The Secretary of Health of the state of Washington declared a pertussis epidemic on April 3, 2012, after the number of reported cases reached 640, compared to 94 cases reported in the same time period in 2011. Pertussis, or “whooping cough,” is a respiratory disease caused by the Bordetella pertussis bacteria. After a person becomes infected, it can take from 7 days to a month for pertussis symptoms to develop. The bacteria cause the disease by releasing toxins that lower the lungs’ ability to clear out respiratory secretions (mucus). After an initial period of a low-grade fever and mild cough, the cough becomes severe and occurs in episodes that prevent the patient from breathing properly, so much so that some patients turn blue during the coughing bouts and for a short time after. According to the Centers for Disease Control and Prevention (CDC), 27,550 cases of pertussis were reported to public health authorities in 2010.
This is not the first time that pertussis has caused outbreaks in the post-vaccine era. In 2010, California reported 9,156 cases, including 10 infant deaths, compared to 965 cases of pertussis in 2009. Between September 2006 and January 2007, a high school in Cook County, Illinois, had to contend with a localized outbreak of pertussis that resulted in 36 cases. Also, between September 2004 and February 2005, 345 cases were reported during an outbreak in an Amish community, with most of those cases being preschool-aged children. In that instance, low vaccination rates were identified as having contributed to the outbreak. These are not the only outbreaks of pertussis.
However, there have also been pseudo-outbreaks of pertussis. These are clusters of people with a diagnosis that is later found to be in error. In Colorado between November 2008 and September 2009, 125 cases of pertussis were reported. Most of these cases were confirmed by PCR (polymerase chain reaction, a test that detects genetic material). After a thorough investigation, epidemiologists determined that the PCR-positive cases were a result of contamination with the vaccine material in the office where the PCR samples were taken. This finding was supported when cultures and blood tests (serology) for B. pertussis and its toxin were negative.
The causes for the situation in Washington are several and varied in nature. Which cause contributed more to the epidemic will be determined at a later time and through careful scientific analysis. However, causes for increases in cases nationwide and within specific areas are known. First, pertussis is known to have periods of high and low activity in 3- to 5-year intervals in the United States. The year 2010 was a peak year, with the overall number of cases reported reaching 27,550, an increase from 2009 (16,858) and 2008 (13,278). So the increase in the number of cases in Washington may be simply reflect the cyclic nature of the disease.
Second, immunity to pertussis wanes both after acquiring the disease or after being vaccinated. This is why a recommendation has been made to give a booster vaccine against pertussis to teens and adults who have not received said booster. It is not unheard of for people’s immune systems to “forget” about the diseases they are immunized against. This is particularly true for diseases that are becoming relatively rare. If the disease is still around, then the person is naturally “boosted” when they are exposed to the disease.
Third, the use of the acellular vaccine (Tdap or DTaP) as a replacement for the whole-cell vaccine (DTP) gives a less-effective immune response because the acellular vaccine contains only parts of the pertussis bacterium. The whole-cell vaccine “presents” the entire bacterium to the immune system. As a result, antibodies would be produced against different parts B. pertussis, offering better “coverage” should the bacteria be encountered again. However, concerns over post-vaccine reactions to the whole-cell vaccine led to the adoption of the acellular vaccine.
Fourth, more and more parents are choosing not to immunize their children against pertussis and other diseases, which creates a larger pool in which pertussis and other vaccine-preventable diseases can incubate and spread. This growing number of non-immune individuals in communities is a threat to herd immunity, where a person is indirectly protected from a disease through the immunity of those around him/her. This translates to more infections among those who are too young, too old, too frail, too sick, or have a medical exemption to being immunized.
Even when many of the cases involved in an outbreak are vaccinated children, unvaccinated children have a higher risk of being involved in the outbreak. For example, in the state of Washington, the most recent data indicate that about 86% of children 19 to 35 months of age are vaccinated with four or more doses of DTaP (the recommended number of doses). For simplicity, let’s assume that there are 1,000 children in this age range in Washington. This means that there would be 860 vaccinated children and 140 unvaccinated children. If there are 100 cases of pertussis in an outbreak among children of that age, and half of those children were found to have been fully immunized, then the odds of being vaccinated and part of the outbreak are 50 out of 860 (5.8%). The odds of being unvaccinated would be 50 out of 140 (35%). That is, children who are not vaccinated have a 6-fold higher risk of being involved in an outbreak.
Finally, this observed rise in the number of reported cases of pertussis may be a result of surveillance bias. This kind of bias happens when health care workers actively search for a disease instead of finding it by chance. In the case of pertussis, a health alert or anecdotal evidence of an outbreak may make healthcare providers more likely to test and diagnose children with a persistent cough as having pertussis. Likewise, laboratory testing for pertussis is becoming easier. While once a bacterial culture was necessary to diagnose a case, now a simple swab of the throat can be used for advanced testing.
Any outbreak of infectious disease is complicated. Infectious diseases require three main elements to spread: a pathogen, a susceptible host, and a means of transmission. But these three ingredients come in many forms. Pathogens are many and have the ability to develop resistance to human immune response and chemical treatment. Susceptible hosts come in the form of the unvaccinated, the very frail, those with underlying medical conditions, and even typical people who never developed immunity even when exposed to the pathogen. All of these elements will need to be effectively understood and managed so that the outbreak of pertussis in Washington can be brought under control.
Additional Sources and Reading
 “Notes from the Field: Pertussis – California, January – June 2010”, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention, available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5926a5.htm. Also Number and Incidence of Vaccine Preventble Diseases-California 2010. Available at http://www.cdph.ca.gov/programs/immunize/Documents/NumberandIncidenceofVaccinePreventableDiseases-California2010.pdf. For number of cases in 2009, see Pertussis Epidemic in California Linked to Vaccination Gaps, American Medical News, 7/26/2012. Available at http://www.ama-assn.org/amednews/2010/07/26/prl10726.htm.
 “Use of Mass Tdap Vaccination to Control an Outbreak of Pertussis in a High School – Cook County, Illinois, September 2006 – January 2007”, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention, available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5729a2.htm
 “Pertussis Outbreak in an Amish Community – Kent County, Delaware, September 2004 – February 2005”, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention, available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5530a1.htm
 “Pertussis Pseudo-outbreak Linked to Specimens Contaminated by Bordetella pertussis DNA From Clinic Surfaces” Pediatrics Vol. 129 No. 2 February 1, 2012, pp. e424 -e430, available at http://pediatrics.aappublications.org/content/129/2/e424.abstract