There has been an increase in the numbers of reported cases of B. pertussis (whooping cough) in the United States in recent decades, especially in the past few years. Reported outbreaks of the highly contagious disease have commonly been attributed to waning protection of the pertussis vaccine and low vaccine coverage.
Additionally, researchers have been arguing about whether the B. pertussis organism started evolving shortly after the whole cell DPT vaccine was given on a mass basis to children in the late 1940s followed by introduction of less reactive acellular DTaP vaccine in the mid 1990s.1 However, now there is wider recognition of scientific evidence that data in recent years points to a fourth cause—asymptomatic transmission of whooping cough by vaccinated individuals.
Reported Whooping Cough Cases Increasing for Decades
In 1922, there were 107,473 reported cases of pertussis in the United States and 5,099 deaths.2 Pertussis-related mortality declined during the 1940s largely due to improved sanitation, hygiene and access to health care. Although there was a general decline in reported pertussis cases after the DPT vaccine was introduced in the late 1940s, reported cases began to increase in the 1980s.3 One 1977 study published by the U.S. Centers for Disease Control and Prevention (CDC) found whole cell DPT vaccine effectiveness was only 63 percent.4
There were 1,010 reported cases of pertussis in the U.S. in 1976 and 25,827 cases in 2004, for a nationwide incidence of 8.5 pertussis cases per 100,000 individuals.5 However, by 2012, there were approximately 48,277 cases of pertussis reported in the U.S.6
Waning Vaccine Effectiveness Faulted for Pertussis Outbreaks
Currently, there are three hypotheses being publicly discussed to explain the reported resurgence of pertussis in the highly vaccinated U.S. population: (1) suboptimal vaccination rates; (2) waning immunity of vaccinated individuals; and (3) evolution of the B. pertussis organism to evade vaccines. The first hypothesis—suboptimal vaccination rates—is not supportable because the U.S. has had a 95 to 98 percent vaccination rate among kindergarten children for the past three decades.7 8
In the U.S., the pertussis vaccine is administered in a combination shot that contains vaccines for diphtheria (D), tetanus (T) and pertussis (P). Due to frequent serious complications, including brain inflammation and permanent brain damage, the older whole cell pertussis vaccine (DPT) that was created in the early 20th century was replaced in 1996 by a purified, less reactive acellular vaccine (DTaP). Although whole cell DPT vaccine is still used in parts of Africa, Asia and the Middle East, children in the U.S., Europe and most other developed countries other countries receive four to five doses of acellular pertussis DTaP vaccine by age six and a booster dose of Tdap vaccine in their adolescent years.9
Evidence suggests that the temporary natural immunity provided by B. pertussis infection and the temporary immunity provided by pertussis vaccines play a role in reported outbreaks of whooping cough.
A 2005 case-control study estimated the effectiveness of DTP and DTaP vaccines to range from 83.6 percent to 97.7 percent, depending on the number of doses administered, vaccine combinations used and age of the child at administration.10
A 2012 analysis of a pertussis outbreak in California revealed that pertussis vaccine effectiveness was 41 percent for children aged two years to seven years, 24 percent for children aged eight years to 12 years and 79 percent among adolescents aged 13 years to 18 years.11 A second 2012 study examining the 2010 pertussis outbreak in California confirmed waning protection from the five-dose DTaP series each year after the final dose.12
A 2013 study backed up these findings by evaluating the protection of the five-dose DTaP series for children born between 1998 and 2003 in Minnesota and Oregon. The incidence of pertussis for the fully vaccinated children rose each year following the fifth dose of the vaccine.13
The CDC issued a press release about the 2012 pertussis outbreak in Washington state acknowledging that pertussis vaccine immunity begins to wane within five years. CDC officials also stated that unvaccinated individuals and children with vaccine exemptions are not responsible for reported whooping cough outbreaks.14
In fact, 73 percent of the children aged seven years to 10 years and 81 percent of the adolescents in Washington with pertussis infections had been fully vaccinated.15
Vaccine Failure Again to Blame
While the evidence for waning vaccine immunity over time has received considerable media attention, relatively few people know that vaccinated persons can become infected and transmit pertussis to others without showing any symptoms.
In a 2014 study, FDA researchers found that non-human primates vaccinated with both whole cell and acellular pertussis vaccines were often protected from severe symptomatic disease but could still become infected and transmit pertussis to close contacts. Although the infant baboons who had received whole cell pertussis vaccine in the study more rapidly cleared the infection and, therefore, transmitted pertussis for a shorter period of time, the baboons given acellular pertussis vaccine also became infected and asymptomatically transmitted disease to close contacts.16
Given this biologically plausible explanation for reported resurgence of pertussis in highly vaccinated human populations, two researchers from Santa Fe Institute, a nonprofit research center, collected empirical data from the CDC on reported pertussis cases in the U.S. from 1922 to 2012. Because of a similar rise in reported cases of pertussis during the past 20 years in the United Kingdom, the New Mexico researchers also collected data on pertussis cases reported in England from 1940 to 2013. These periods accounted for a time when there was no pertussis vaccination and times when whole-cell and acellular pertussis vaccines were used. Coupled with genomic data, the investigators created a detailed epidemiological model of whooping cough transmission.
Their conclusion, which they published in 2015, was that vaccinated individuals may not display any symptoms of whooping cough, but can still be infected with the bacterium and unknowingly spread the disease multiple times in their lifetime. This asymptomatic transmission not only contributed to the observed increase in pertussis incidence over the years, but may have exacerbated it.
Further, the model showed that vaccinating parents, siblings and other individuals in close contact with unvaccinated infants too young to receive pertussis vaccine—a theoretical method of protection that public health officials call “cocooning” may be completely ineffective. The researchers also pointed out that the theory of achieving “herd immunity,” which is supposed to be accomplished when a high enough proportion of individuals in a population have been vaccinated in order to protect unvaccinated individuals, was also impractical for pertussis vaccine according to the model.
“Clearly, more research is necessary, but if our results hold, public health authorities may be facing a situation similar to that of polio, where vaccinated individuals can still transmit infection,” the researchers wrote.
In light of current evidence and our results, we cannot dismiss the potential far-reaching epidemiological consequences of asymptomatic transmission of B. pertussis and an ineffective B. pertussis vaccine.17
As NVIC co-founder and president Barbara Loe Fisher commented earlier this year,
When there are a lot of people with silent asymptomatic pertussis infections, it is impossible to know who is a carrier and who is not, which means that reported cases of pertussis are just the tip of a very big iceberg. It also means that articles blaming whooping cough cases on unvaccinated or partially children are nothing more than wishful thinking and scapegoating. If vaccinated people can get silently infected and transmit infection without showing any symptoms—even after getting four to six pertussis shots—then pertussis vaccine acquired ‘herd immunity’ is an illusion and always has been.18
1 Fisher BL. Pertussis Microbe Outsmarts the Vaccines as Experts Argue About Why. NVIC Newsletter Mar. 27, 2016.
2 U.S. Centers for Disease Control and Prevention. Achievements in Public Health, 1900-1999 Impact of Vaccines Universally Recommended for Children—United States, 1990-1998. Mortality and Morbidity Weekly Report 1999;48:243-8.
3 U.S. Centers for Disease Control and Prevention. Pertussis (Whooping Cough) Cases by Year (1922-2014). CDC.gov Sept. 8, 2015.
4 Broome CV, Preblud SR, Bruner B et al. Epidemiology of pertussis, Atlanta, 1977. J Pediatr 1981; 98(3): 362-267.
5 Tiwari T, Murphy TV, Moran J. Recommended Antimicrobial Agents for the Treatment and Postexposure Prophylaxis of Pertussis: 2005 Guidelines. Morbidity and Mortality Weekly Report 2005; 54(RR14): 1-16.
6 CDC. About Pertussis Outbreaks. CDC.gov Sept. 8, 2015 (updated).
7 Hinman A, Orenstein WA, Schuchat A. Vaccine Preventable Diseases, Immunization and MMWR 1961-2011. MMWR Oct. 7, 2011; 60(04): 49-57.
8 CDC. Vaccination Coverage Among Children in Kindergarten – United States, 2014-15 School Year. MMWR Aug. 28, 2015: 64(33): 897-904.
9 National Vaccine Information Center. Pertussis (Whooping Cough) & Pertussis Vaccine. NVIC.org July 5, 2016 (accessed).
10 Bisgard KM, Rhodes P, Connelly BL, et al. Pertussis Vaccine Effectiveness Among Children 6 to 59 Months of Age in the United States, 1998-2001. Pediatrics 2005;116:e285-94.
11 Witt MA, Katz PH, Witt DJ. Unexpectedly Limited Durability of Immunity Following Acellular Pertussis Vaccination in Preadolescents in a North American Outbreak. Clinical Infectious Diseases 2012;54:1730-5.
12 Misegades LK, Winter K, Harriman K. Association of Childhood Pertussis With Receipt of 5 Doses of Pertussis Vaccine by Time Since Last Vaccine Dose, California, 2010. The Journal of the American Medical Association 2012;308:2126-32.
13 Tartof SY, Lewis M, Kenyon C. Waning Immunity to Pertussis Following 5 Doses of DTaP. Pediatrics March 11, 2013.
14 CDC. Pertussis Epidemic in Washington State. CDC. gov (press release) July 19, 2012.
15 Moyer MW. The Broken Vaccine. Discover Feb. 18, 2013.
16 Warfel JM, Zimmerman LI, Merkel TJ. Acellular pertussis vaccines protect against disease but fail to prevent infection and transmission in a nonhuman primate model. Proceedings of the National Academy of Sciences 2014;111:787-92.
17 Althouse BM and Scarpino SV. Asymptomatic transmission and the resurgence of Bordetella pertussis. BioMed Central 2015;146.
18 Fisher BL. Pertussis Microbe Outsmarts the Vaccines as Experts Argue About Why. NVIC Newsletter Mar. 27, 2016.