Wednesday, October 15, 2014

Isolation and quarantine

I started this blog nearly three years ago. Yesterday it broke 10,000 pageviews. That may not be much compared to more popular blogs, but I'm glad to know that someone is reading what I write.


I don't have the raw data, but this graph looks like a regression line would trend upward.

I don't expect this post to get a lot of hits.

Our office has received a lot of calls in the last few weeks from health care providers who are unsure of or confused about the recommendations for monitoring people who may have been exposed to Ebola virus, so I suspect that there is similar uncertainty and confusion among the general public.

For this discussion, I'm talking about people who have been in direct contact with a person with Ebola virus disease (EVD); that is, people who have potentially been exposed to and may have been infected with the virus but have no symptoms of the disease.

Also, I'm writing about the current recommendations for contacts of cases. These recommendations may change. Please refer to the Center for Disease Control and Prevention (CDC) website for changes in its recommendations.

The CDC does not recommend isolating people who have been in contact with a person with EVD but have no symptoms of the disease. A person who is infected with the virus but has no symptoms of the disease cannot infect other people. According to the current CDC guidelines, health care workers who have had contact with someone with EVD may return to work. Of course, that changes once a person develops a fever and symptoms of EVD.

The CDC recommends that contacts of cases take their temperature twice every day for 21 days (the longest incubation period for EVD) and notify their local health authority of fever or symptoms of EVD. Health care workers may be monitored by their employers.

We are currently working with the Washington State Department of Health, other local health jurisdictions, and hospitals to develop policies and procedures to monitor health care workers who have potentially been exposed to Ebola. This will include those of us who return from working in West Africa.

One thing to remember is that the CDC is not a regulatory agency. In general, the CDC can make recommendations and serve in advisory capacity, but it has no authority to enforce its recommendations.

Isolation and quarantine

The words isolation and quarantine are sometimes used interchangeably but, in public health, they do not have the same meaning. The CDC defines isolation as separating sick people with a contagious disease from people who are not sick. Quarantine is separating and restricting the movement of people who were exposed to a contagious disease to see if they become sick.

For example, I am a tuberculosis case manager. We routinely ask people who have tuberculosis and are contagious to isolate themselves to avoid infecting other people. Because tuberculosis is not transmitted outdoors, we do not tell a person with pulmonary tuberculosis that she or he must remain indoors. That person may leave the home but may not enter another building where other people could be exposed to tuberculosis. This is called voluntary isolation.

Washington State law gives local (county) health officers the authority to order a person into isolation. If that person fails to comply with the health officer order, we may get a court order for involuntary isolation. Washington State health officers have the authority to restrict the movement of (quarantine) individuals who are a threat to public safety. Fortunately, I've never had to do more than explain that the health officer has that authority.

The CDC has the authority to detain individuals who are entering the U.S. or traveling across state lines, but it does not have the authority to quarantine individuals within the boundaries of states or local health jurisdictions. State and local health officers have that authority.

I know that a lot of people are worried about Ebola in the U.S. I am also concerned about the risk of this epidemic spreading beyond the borders of Guinea, Liberia, and Sierra Leone to neighboring countries and out of Africa. That is why I decided to work in West Africa to help get people with EVD into treatment and end this epidemic.

References:

Centers for Disease Control and Prevention. (2014). Infection prevention and control recommendations for hospitalized patients with known or suspected Ebola virus disease in U.S. Hospitals. http://www.cdc.gov/vhf/ebola/hcp/infection-prevention-and-control-recommendations.html.

Centers for Disease Control and Prevention. (2014). Interim guidance on monitoring and movement of persons with Ebola virus disease exposure. http://www.cdc.gov/vhf/ebola/hcp/monitoring-and-movement-of-persons-with-exposure.html.

Centers for Disease Control and Prevention. (2014). Legal authorities for isolation and quarantine. http://www.cdc.gov/quarantine/aboutlawsregulationsquarantineisolation.html.

Washington State Legislature. (n.d.) Conditions and principles for isolation and quarantine. http://app.leg.wa.gov/wac/default.aspx?cite=246-100-045.

Washington State Legislature. (n.d.) Tuberculosis – prevention, treatment, and control. http://app.leg.wa.gov/wac/default.aspx?cite=246-170.

 

Sunday, October 12, 2014

Is Ebola airborne?*

Regarding my upcoming trip to Liberia, I suspect that there are people who are concerned about not only my safety, but also the safety of my family and my community. I adore my wife and son and would not have chosen to work as an Ebola Response Clinician unless I believed that I could do so safely and without leaving Holly without a husband and Andrew without father or, unthinkably, exposing either of them to a highly lethal virus.

 
From hearing people's comments in the media and from some of the phone calls we have received at the health department, it seems to me that there is a lot of confusion about the means by which ebolaviruses are transmitted. Although I addressed Ebola transmission in the presentation I gave to the Pierce County Medical Reserve Corps, I'd like to go into more detail about the question of whether ebolaviruses are transmitted by the airborne route.

The Centers for Disease Control and Prevention, the World Health Organization, and every textbook chapter on Ebola virus disease (EVD) that I have read acknowledge that there is a risk of ebolavirus transmission through respiratory droplets, which are expelled when a person coughs or sneezes. At the same time, all of those sources state that there is no evidence that ebolaviruses are transmitted from person-to-person by the airborne route. That may seem like a contradiction, but the difference is in the size of the droplets.

Coughing and sneezing generates large respiratory droplets that fall out of the air within a few feet. Transmission through large droplets usually requires close contact. These droplets can come in contact with the eyes or be inhaled and trapped in the upper airway: the nose, mouth, pharynx, trachea, and bronchi.


NCI, 2012
Pertussis and influenza are examples of diseases that are transmitted by large respiratory droplets. These large droplets can be blocked by wearing a simple surgical mask – the type you can buy at your local drug store.

Airborne transmission refers to pathogens that are carried by droplet nuclei, particles that are 1 to 5 micrometers (μm) in diameter. These particles can remain suspended in the air for hours and can pass through the upper airway into the alveoli.

NCI
Droplet nuclei are not blocked by surgical masks and require high-efficiency particulate air filtration (HEPA) for protection. In a health care setting, these are usually N95 masks or a powered air-purifying respirator (PAPR). Mycobacterium tuberculosis, the bacteria that causes tuberculosis, is an example of a pathogen that is transmitted by the airborne route.
Powered air-purifying respirator (PAPR)
CDC
Measles is another example of a disease that is transmitted by the airborne route. This year in Pierce County we had two public measles exposures that required the health department to notify the public of where and when a person with measles had been.
Because the small particles that transmit measles can remain suspended in the air for hours, we recommend that people who were in a place at the time a person was measles was there and two hours after either ensure that they are either immune to measles or receive a dose of MMR.

Another important difference between measles and ebolaviruses is that people with measles are contagious up to 4 days before showing symptoms of the disease. A person with EVD cannot transmit the virus until she or he is symptomatic.

A number of people have raised concerns over the adequacy of surgical masks to prevent health care workers frombeing infected with Ebola. Adding to the confusion, it appears that there is airborne transmission of Reston ebolavirus (which does not cause disease in humans) between non-human primates and Zaïre ebolavirus has been transmitted between non-human primates under experimental conditions. Michael Osterholm raised the question of whether Zaïre ebolavirus, the species causing the current epidemic in West Africa, could mutate to become airborne.

It is very distressing that several health care providers working in the Ebola epidemic in West Africa have become infected and developed EVD. We don't know the circumstances under which they became infected. It's important to realize that there are hundreds of expatriate health workers in West Africa so, although it certainly gives us reason for concern, those providers are a small proportion of the total those who have worked and continue to work in clinical settings in West Africa.

As I mentioned in my last post, my first stop will be Atlanta to receive safety training at the CDC. Once I return to the U.S., I will monitor my temperature twice daily and report any symptoms suggestive of EVD to the local health authority; that is, the very office where I work!

I intend to take every precaution to protect myself, my family, and my community.

I appreciate the support and prayers from my family, friends, and colleagues.

I encourage other health care providers to consider volunteering your time to help end this epidemic.
References

Blumberg, L., Enria, D., & Bausch, D. G. (2014). Viral hemorrhagic fevers. In J. Farrar, P. J. Hotez, T. Junghanss, G. Kang, D. Lalloo, & N. J. White (Eds.) Manson's tropical diseases, 23rd Ed. [Electronic version]. Elsevier.

Brosseau, L. M. & Jones, R. (Sept. 17, 2014). Commentary: Health workers need optimal respiratory protection for Ebola. http://www.cidrap.umn.edu/news-perspective/2014/09/commentary-health-workers-need-optimal-respiratory-protection-ebola.

Centers for Disease Control and Prevention. (2014). Infection prevention and control recommendations for hospitalized patients with known or suspected Ebola virus disease in U.S. hospitals. http://www.cdc.gov/vhf/ebola/hcp/infection-prevention-and-control-recommendations.html.

Geisbert, T. W. (2014). Marburg and Ebola hemorrhagic fevers (Filoviruses). In J. E. Bennett, R. Dolin, & M. J. Blaser (Eds.). Mandell, Douglas, and Bennett's principles and practice of infectious diseases, 8th Ed. [Electronic version]. Elsevier.

Hartman, A. L. (2013). Ebola and Marburg virus infections. In, A. J. Magill, D. R. Hill, T. Solomon, & E. T. Ryan (Eds.) Hunter's tropical medicine, 9th Ed. [Electronic version]. Elsevier.

Osterholm, M. T. (Sept. 11, 2014). What we're afraid to say about Ebola. http://www.nytimes.com/2014/09/12/opinion/what-were-afraid-to-say-about-ebola.html.

World Health Organization. (2014). What we know about transmission of the Ebola virus among humans. http://www.who.int/mediacentre/news/ebola/06-october-2014/en.

*Betteridge's law: Any headline which ends in a question mark can be answered by the word no.

 

Wednesday, October 8, 2014

Ebola virus disease

I never finished unpacking from my trip to Tanzania.

A couple of weeks ago I received an Email from the American Society of Tropical Medicine and Hygiene that included contact information for several organizations that are recruiting nurses and doctors to work in the Ebola epidemic response in West Africa. My first thought was that I couldn't do it, I couldn't leave Holly and Andrew again, I can't afford to leave work for an extended period of time. I jokingly forwarded the Email to Holly and my supervisor expecting a chorus of, "No!"

At some point that day, I couldn't think of anything else.

I sat down with Holly and told her what I had been thinking. This is what is needed to end this epidemic. This is why I became a nurse. This is why I got a degree in tropical medicine. I want to set an example for my son. I thought she would object. She didn't. Holly has been very supportive. She has prayed that this would work out.

It has. I'm going to Liberia.

I've been offered a temporary position with Partners In Health. In a few weeks I'll go to Atlanta for training at the CDC, then I'll receive two weeks training in Liberia, and then I'll spend four weeks at my worksite.


The hardest thing about being in Tanzania for a month was being away from my little boy. The hardest part of making my decision  to go to Liberia is being away from Andrew for even longer than my trip to Tanzania. I'll be able to Skype Holly and Andrew while I'm in Liberia, which will make that a little easier. I plan to make spending time with Andrew my priority between now and the time I leave for Atlanta.

This is a presentation on Ebola virus disease that I gave to the Pierce County Medical Reserve Corps on Monday:
 
 

Monday, September 8, 2014

Home

I left Tanzania last Saturday evening, arrived home Sunday afternoon, and was back at work Tuesday morning. Since I returned home a week ago, Andrew has taken a lot of time out of his busy schedule to hang out with me.

This was my fourth trip to Africa, my third trip as a nurse, but my first trip as a husband and father. It was also the first time I had spent more than a week away from home since Andrew was born. Fortunately, I had access to a high-speed Internet connection and was able to Skype Holly and Andrew almost every day.
 
Andrew and his granddad Rollosson talking to me via Skype
It's interesting to consider that during my first trip to Africa in the late 1980s, most people in the U.S. had not heard of the Internet. When I was in Ethiopia, we had to drive an hour to the next city to plug our laptop into a phone line so we could send and receive Email over a dial-up connection. Now, I not only had high-speed Internet in a rural area of sub-Saharan Africa, cell phones seem to be ubiquitous.

Being able to talk to Holly and Andrew and hear my little boy laugh made the separation easier for me but, after a month, all I wanted to do was come home to my wife and child. I think Andrew got tired of the daddy-on-the-computer routine. The last week or so he would look for me behind the monitor and under the table.

I left bedside nursing for public health five years ago. The focus of my work shifted from helping people recover from injuries and illness to preventing illness. In addition to stepping back into my role as a neuroscience nurse, I was also working in a setting where there were inadequate resources to care for severely injured patients and where I expected to see people die who might have survived if they had received care in the U.S. I knew that I wouldn't be able to fix all of the problems that I saw and that the best I could do was share what I know with my Tanzanian colleagues so that they are better able to care for their patients.
 
Teaching the Glasgow Coma Scale to student nurses
I gave a presentation on care of patients with spinal cord injuries the day before I left Haydom. I began my talk by congratulating the hospital staff for providing high-quality care to their community. I ended my presentation with Proverbs 27:17, "As iron sharpens iron, so one person sharpens another," and thanked them for sharpening me.

My work at Haydom was challenging, but I'm glad that I had this opportunity. I will do it again, and I encourage my colleagues to seek opportunities to work in a developing country.
 
 
Out of all of the health workers I khave known who have worked in Africa, I can't think of one who has been there only once. There is something about working in Africa that pulls us back.
 
 
Africa is full of surprising beauty. Even in the semi-arid climate of East Africa, there is beauty in the iridescent colors that seem to spring unexpectedly from the landscape. More importantly, there is the beauty of the human spirit; the joy of living in spite of hardship.
 

Outreach: I spent the day in the shade of acacia trees weighing babies and checking pregnant women's blood pressure
Spending time away from the noise and distractions and the pace of living in a wealthy industrialized country gives me an opportunity to see the world from a different perspective. I return home with a recognition of the triviality of many of the things that occupy our minds and our time and an appreciation that some of the things for which we have the luxury of taking for granted are what is truly precious.


Friday, August 8, 2014

Greetings from Tanzania!

I am currently teaching nurses at Haydom Lutheran Hospital in Tanzania.

Although this is my first trip to Tanzania, this is my fourth trip to Africa and my third time on the continent working as a nurse. I decided to become a nurse after my first trip to Africa in 1988-89.

I think it's easy to come to developing country and see only poverty and suffering. It's easy to feel frustration and a sense of futility when confronted with the problems people living in an impoverished part of the world face. Here, problems that are relatively rare in the U.S. are common. I have seen expatriate health care workers become pessimistic and angry when simple solutions to problems seem to be unavailable in rural Africa.

I am not immune to those feelings, however, I also see here resourcefulness in a resource-poor setting, the beauty of humanity and Creation, and, most importantly, hope.

Here, a mother of ten children can have surgery to repair her torn uterus and receive a safe blood transfusion rather than bleed to death or be infected with a blood-borne pathogen,

Unit of blood tested for HIV, hepatitis B, and hepatitis C

 a person with a spinal cord injury or a stroke can learn to walk again,


myelomeningoceles are repaired,


tuberculosis is cured,



and broken bones are repaired.



Here, people who would have died without the medical and surgical services provided survive, disabilities are minimized, and people return home to their families.

My special thanks to Karen March, Neuroscience Clinical Nurse Specialist, mentor, and friend, who facilitated my visit to Haydom Lutheran Hospital.


Karen buying a SIM card for her cell phone from a vendor in Haydom


Sunday, July 27, 2014

Demographic transition: do vaccines reduce fertility?


First, what is fertility? To most people, fertility is the ability to have children; the opposite of infertility. To demographers and epidemiologists, fertility is the number of children a person has. According the U.S. Census Bureau, fertility is the number of children ever born to a person (referring to the number of live births). "Typically it is asked of women age 15 to 50, or women of all ages but some surveys ask men how many children they have fathered." A fertility rate is the number of children born in a population over a period of time, usually per 1,000 people per year. To demographers and epidemiologists, the ability to have children in known as fecundity rather than fertility.

In his book, The End of Poverty, Jeffery Sachs wrote, "I have been asked dozens of times if help for Africa would ultimately backfire in an even greater population explosion. Would greater child survival rates not translate into more adult hunger and suffering?"

I've been asked similar questions about my interest in tropical medicine and my volunteer work in Africa. Sachs goes on to discuss the demographic transition, a phenomenon that has occurred in every industrialized country and is occurring in developing countries; that is, as standards of living improve, people have fewer children.

There are a several theories about why the demographic transition occurs. The primary theory links decreasing fertility to decreases in child mortality; parents have fewer children when more of their children survive to adulthood.

We can see this occurring today. In the graph below, I've used World Bank data from 2012 to plot child mortality rates (the number of deaths of children under 5 years of age per 1,000 population per year) against fertility rates. Each dot represents a country. In countries in which child mortality is low, women have fewer children. In countries with high death rates for children under 5 five years of age, fertility rates are higher.

 

Childhood vaccines are associated with improved child survival. Measles immunization has been shown to decrease all cause mortality. In the graph below I've plotted mortality of children less than 5 years of age against the percent of children 12 to 23 months of age who have received measles vaccines. Under 5 mortality is higher in countries with lower measles immunization coverage than in countries with high measles vaccine coverage.

 

Of course, there are a number of other factors associated with both decreasing child mortality and decreasing fertility including higher costs of raising a child (e.g., education), transitioning from agricultural to manufacturing markets and urbanization (people who live in cities tend to have fewer children than those living in rural areas), more women working outside of the home, changes in social norms, and, of course, access to contraception.

Although there have been improvements in the prevention and treatment of malaria, most malaria deaths occur in children under 5 years in sub-Saharan Africa. This is one of the reasons fertility remains high in Africa.

In 2010, Bill Gates made a passing reference to the demographic transition during a TED Talk on energy and climate:

First, we've got population. The world today has 6.8 billion people. That's headed up to about nine billion. Now, if we do a really great job on new vaccines, health care, reproductive health services, we could lower that by, perhaps, 10 or 15 percent, but there we see an increase of about 1.3.

Gates had discussed the demographic transition in the Bill and Melinda Gates Foundation 2009 annual letter:

Two things caused this huge reduction in the death rate. First, incomes went up, and with that increase, nutrition, medical care, and living conditions improved. The second factor is that even where incomes did not go up, the availability of life-saving vaccines reduced the number of deaths. For example, measles accounted for 4 million children’s deaths in 1990, but fewer than 250,000 in 2006.

A surprising but critical fact we learned was that reducing the number of deaths actually reduces population growth. Chart 3 shows the strong connection between infant mortality rates and fertility rates. Contrary to the Malthusian view that population will grow to the limit of however many kids can be fed, in fact parents choose to have enough kids to give them a high chance that several will survive to support them as they grow old. As the number of kids who survive to adulthood goes up, parents can achieve this goal without having as many children.


Bill and Melinda Gates Foundation, 2009

Unfortunately, Gates' TED Talk comments were misinterpreted by several people in the blogosphere to mean that vaccines cause infertility.

To answer the question in the title of this post, "do vaccines reduce fertility?" I will violate Betteridge's law of headlines and say, yes, vaccines reduce fertility, but not in the way some people would like you to believe.

 
References:

Bloom, D. E., Canning, D., & Weston, M. (2005). The value of vaccination. World Economics, 6(3), 15-39.

Brauner-Otto, S., Axinn, W., & Ghimire, D. (2007). The spread of health services and fertility transition. Population Studies Center Research Report 07-619. http://www.psc.isr.umich.edu/pubs/pdf/rr07-619.pdf.

Conley, D., McCord, G. C., & Sachs, J. D. (2007). Africa's lagging demographic transition: evidence from exogenous impacts of malaria ecology and agricultural technology. National Bureau of Economic Research Working Paper Series No. 12892. http://www.nber.org/papers/w12892.

Gates, B. (2009). Bill and Melinda Gates Foundation annual letter 2009. http://www.gatesfoundation.org/who-we-are/resources-and-media/annual-letters-list/annual-letter-2009.


Greenwood, J. & Sesharid, A. (2001). The U.S. demographic transition. AEA Papers and Proceedings, 92(2), 153-159. http://www.econ.wisc.edu/~aseshadr/publication_pdf/usdt.pdf.

Newson, L., Postmes, T., Lea, S. E. G., & Webley, P. (2005). Why are modern families small? Toward an evolutionary and cultural explanation for the demographic transition. Personality and Social Psychology Review, 9(4), 360-373.

Omran, A R. (1971). The epidemiologic transition. Milbank Memorial Fund Quarterly, 49(4), 509-538.

Sachs, J. D. (2005). The end of poverty: economic possibilities for our time. New York: The Penguin Press.

van den Ent, M. M. V. X., Brown, D. W., Hoelstra, E. J., Christie, A., & Cochi, S. L. (2011). Measles mortality reduction contributes substantially to reduction of all cause mortality among children less than five years of age, 1990-2008. Journal of Infectious Diseases, 204(Supple. 1), S18-S23. http://jid.oxfordjournals.org/content/204/suppl_1/S18.long.

World Bank. (2014). Data. http://data.worldbank.org.
 

 

Sunday, July 13, 2014

Modified measles


Happy birthday Andrew!
 

 
Our little boy is two years old today. Seeing Andrew grow up has been one of the greatest joys of my life. I have been enthralled by watching our little scientist learn about the world. Watching him learn new skills and challenge himself has made me a very proud daddy!


Measles vaccine is highly effective; so effective that high immunization coverage has resulted in the elimination of measles from the U.S. Nevertheless, a small proportion of people who receive measles vaccine do not develop an immune response sufficient to prevent infection. This is known as primary vaccine failure. There is evidence that measles antibody levels (titers) wane over time allowing some people to become susceptible to measles. Infection in a person who initially developed an adequate immune response to the vaccine but later became susceptible is called secondary vaccine failure.

One of the factors that contributes to waning immunity to measles is the loss of natural boosting. In the past, people in the U.S. were periodically exposed to measles virus during epidemics. Since measles has been eliminated from the U.S., exposure to the virus in this country has become rare.

Secondary measles vaccine failure is more likely to occur with intense exposure to the measles virus. This can occur during an outbreak or among household contacts of a person with measles. For this reason, it's not unusual to find vaccinated people with measles during an outbreak.

Modified measles, measles in a vaccinated person, is much milder than measles in an unvaccinated person; fever and rash are less severe and a person with modified measles is much less likely to develop complications of measles than an unvaccinated person with measles. In contrast to unvaccinated people with measles, vaccinated people who develop modified measles have high avidity antibodies, meaning, their antibodies are mature and bind tightly to the virus.

People with modified measles are also much less likely to infect other people than unvaccinated people with measles. There have been numerous cases of modified measles in previously vaccinated people with no evidence of transmission to close contacts reported in the medical literature. There have even been reports of vaccinated doctors who developed modified measles but did not infect their patients (Lee et al., 2008; Rota et al., 2011).

Earlier this year, Jennifer Rosen and colleagues published the first report of transmission of measles from a previously vaccinated person. In 2011, a 22-year-old woman in New York City who had received two doses of MMR as a child who developed measles. Out of 88 people who had contact with her, 4 people developed measles. One was her coworker and the other three were health care workers at a clinic. Two of the secondary cases had received two doses of MMR as children and the other two had previous evidence of immunity (positive immunoglobulin G titer). None of the contacts to the four secondary cases developed measles. The authors wrote, "this outbreak probably represents a series of rare events" and that it "does not justify a change in current measles control and elimination strategies."

References:

Centers for Disease Control and Prevention. (2013). Prevention of measles, rubella, congenital rubella syndrome, and mumps, 2013: summary recommendations of the Advisory Committee on Immunization Practices (ACIP). Morbidity and Mortality Weekly Report, 62(4), 1-34. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr6204a1.htm.

Gershon, A. A. (2009). Measles virus (rubeola). In G. L. Mandell, J. E. Bennett, & R. Dolin (Eds.). Mandell, Douglas, and Bennett’s principles and practice of infectious diseases. (7th Ed.) [Electronic version].

Hickman, C. J., Hyde, T. B., Sowers, S. B., Mercader, S., McGrew, M., Williams, N. J. et al. (2011). Laboratory characterization of measles virus infection in previously vaccinated and unvaccinated individuals. Journal of Infectious Diseases, 204(Suppl. 1), S549-S558. http://jid.oxfordjournals.org/content/204/suppl_1/S549.full.

Lee, N. Y., Lee, H. C., Chang, C. M., Wu, C. J., Ko, N. Y., Ko, C. (2008). Modified measles in a healthcare worker after return from travel. Infection Control and Hospital Epidemiology, 29(4), 380-381. http://www.jstor.org/stable/10.1086/529031.

Mercader, S., Garcia, P., & Bellini, W. J. (2012). Measles virus avidity assay for use in classification of measles vaccine failure in measles elimination settings. Clinical and Vaccine Immunology, 19(11), 1810-1817. http://cvi.asm.org/content/19/11/1810.

Mitchell, P., Turner, N., Jennings, L., & Dong, H. (2013). Previous vaccination modifies both the clinical disease and immunological features in children with measles. Journal of Primary Health Care, 5(2), 93-98. http://www.ncbi.nlm.nih.gov/pubmed/23748389.

Rosen, J. B., Rota, J. S., Hickman, C. J., Sowers, S. B., Mercader, S., Rota, P. A. et al. (2014). Outbreak of measles among persons with prior evidence of immunity, New York City, 2011. Clinical Infectious Diseases, 58(9), 1205-1210. http://cid.oxfordjournals.org/content/early/2014/02/27/cid.ciu105.

Rota, J. S., Hickman, C. J., Sowers, S. B., Rota, P. A., Mercader, S., & Bellini, W. J. (2011). Two case studies of modified measles in vaccinated physicians exposed to primary measles cases: high risk of infection but low risk of transmission. Journal of Infectious Diseases, 204(Suppl. 1), S559-S563. http://jid.oxfordjournals.org/content/204/suppl_1/S559.full.