Update: Washington State pertussis epidemic

Andrew received his first dose of hepatitis B vaccine minutes after he was born. My intention had been to write an entry on hepatitis B but, as you can see, I've had my hands full.

In April of this year, Washington State Health Secretary Mary Selecky declared that pertussis (whooping cough) had reached epidemic level in the state. At that time, 640 cases had been reported to the state Department of Health since the beginning of the year.

Last Friday (July 20, 2012), the Centers for Disease Control and Prevention (CDC) published a summary of the current pertussis epidemic in Washington State in the Morbidity and Mortality Weekly Report (MMWR). Between January 1^st and June 16^th, 2,520 cases had been reported – 13 times the number reported during the same time last year. Illinois, New York and Wisconsin have also had dramatic increases in the numbers of pertussis cases compared to last year. In fact, nearly half of all the states have reported more than double the numbers of cases in the first six months of 2012 than their total numbers of cases in 2011 (CDC, 2012a).

Overall, the national incidence of pertussis is increasing. In 2011, a total of 8,284 cases of pertussis were reported in the U.S. So far this year, 17,920 cases have been reported. One notable exception is California. A total of 7,195 cases of pertussis were reported during the 2010 epidemic in California. Last year 1,709 cases were reported in California and, so far this year, 275 cases have been reported.

I included this graph in my previous post on pertussis vaccines:

CDC, 2002

What we see is a sharp decrease in the incidence of pertussis after the introduction of whole-cell pertussis vaccine in the mid-1940's to the lowest point in 1976. Since then, there has been a gradual increase in pertussis incidence.

There are a number of factors that could be contributing to the increase in the number of pertussis cases. One is simply that pertussis is being recognized and diagnosed more frequently. Pertussis used to be thought of as a childhood disease and immunity to pertussis was believed to be life-long. We now know that immunity to pertussis wanes and that adults usually don’t have the severe symptoms that are seen in children.

The epidemiological data from the current pertussis epidemic in Washington State and the 2010 California epidemic suggests that immunity to pertussis from acellular pertussis vaccines wanes more quickly than immunity from whole-cell pertussis vaccines, which are no longer used in the U.S.

Whereas acellular pertussis vaccines contain up to 5 antigens, whole-cell pertussis vaccines contained around 3,000 antigens. Whole-cell vaccines are highly effective; unfortunately, they also have a high occurrence of adverse events ("side effects"). Acellular vaccines have fewer side effects and appear to be highly effective for the first two years after immunization, but immunity seems to wane more quickly than after immunization with a whole-cell pertussis vaccine.

So, why should my child or I receive a pertussis vaccine if I can still get pertussis anyway? There are several reasons. Even though most of the cases of pertussis are in vaccinated people, the attack ratio is much higher in unvaccinated people. Unvaccinated children are eight times more likely to get pertussis than children who have received all of the recommended doses of DTaP (CDC, 2012b). Unvaccinated children with pertussis are more likely to have severe disease, cough longer, and infect other people than vaccinated children (Baptista et al, 2006; Préziosi & Halloran, 2003; Tozzi et al., 2003).

No vaccine is 100% effective. We have known that immunity to pertussis wanes, but it now appears that immunity from acellular vaccines wanes more quickly than we thought. Nevertheless, acellular pertussis vaccines remain our best defense against pertussis.

On a personal note, one of my duties as a nurse epidemiologist in a county health department is to investigate cases of notifiable diseases. State law requires health care providers to report notifiable diseases to their local health jurisdiction (LHJ). Pertussis is one of those diseases. When the health department receives notification of a case of pertussis, one of my colleagues or I contact the parent of the child or the person with pertussis and complete a case report form. We then submit the data that we collect to the Washington State Department of Health and from there it is sent to the CDC. In this case, the CDC published their analysis of Washington State pertussis data in the MMWR cited above. In other words, the CDC report includes data that I collected.

References:

Águas, R., Gonçalves, G., & Gomes, G. M. G. (2006). Pertussis: increasing disease as a consequence of reducing transmission. Lancet Infectious Diseases, 6(2), 112-117. doi.org/10.1016/S1473-3099(06)70384-X.

Baptista, P. N., Magalhães, V., Rodrigues, L. C., Rocha, M. A. W., & Pimentel, A. M. (2006). Pertussis vaccine effectiveness in reducing clinical disease, transmissibility, and proportion of case with positive culture after household exposure in Brazil. Pediatric Infectious Disease Journal, 25(9), 844-846.

Centers for Disease Control and Prevention. (2002). Pertussis – United States, 1997-2000. Morbidity and Mortality Weekly Report, 51(4), 73-76. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5104a1.htm.

Centers for Disease Control and Prevention. (2012). Notifiable diseases and mortality table. Morbidity and Mortality Weekly Report, 61(28), ND-382-ND-395. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6128md.htm.

Center for Disease Control and Prevention. (2012). Pertussis epidemic – Washington, 2012. Morbidity and Mortality Weekly Report, 61(28), 517-522. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6128a1.htm.

Offit, P. A., Quarles, J., Gerber, M. A., Hackett, C. J., Marcuse, E. K., Kollman, T. R. et al. (2002). Addressing parents’ concerns: do multiple vaccines overwhelm or weaken the infants immune system? Pediatrics, 109(1), 124-129. http://pediatrics.aappublications.org/content/109/1/124.full.

Préziosi, M-P. & Halloran, E. (2003). Effects of pertussis vaccination on disease: vaccine efficacy in reducing clinical severity. Clinical Infectious Diseases, 37(6), 772-779. http://cid.oxfordjournals.org/content/37/6/772.long.

Rohani, P. & Drake, J. M. (2011). The decline and resurgence of pertussis in the US. Epidemics, 3(3-4), 183-188. doi: 10.1016/j.epidem.2011.10.001.

Tozzi, A. E., Ravá, L., Ciofi degli Atti, M. L., Salmaso, S., Progetto Pertosse Working Group. (2003). Clinical presentation of pertussis in unvaccinated and vaccinated children in the first six years of life. Pediatrics, 112(5), 1069-1075. http://pediatrics.aappublications.org/content/112/5/1069.full

Witt, M. A., Katz, P. H., & Witt, D. J. (2012). Unexpectedly limited durability of immunity following acellular pertussis vaccination in preadolescents in a North American outbreak. Clinical Infectious Diseases, 54(12), 1730-1735. doi: 10.1093/cid/cis287.