DTaP Study Controversy: Asymptomatic Carriers

The wording of this study has many people confused about whether the DTaP/ TDaP vaccines shed. Vaccine shedding (more on this here) occurs when a live virus vaccine itself causes the recipient to come down with the illness the vaccine is meant to prevent. This has been most commonly seen with with nasal flu vaccine, and up to 10% of those who received the MMR come down with a measles rash. (sources and sources)

Summary of the Study:

We hypothesized an additional explanation for pertussis resurgence is that aP-vaccinated individuals can act as asymptomatic or mildly symptomatic carriers and contribute significantly to transmission in the population. Observational studies suggest that asymptomatic pertussis can occur in vaccinated children and adults based on PCR or serological data (34, 35). However, during the aP vaccine trials, participants were not screened for B. pertussis infection unless they presented with pertussis-like symptoms and at least 7–21 d cough (12). Therefore, no experimental data exist on whether vaccination prevents B. pertussis colonization or transmission in humans. In the present study we show that aP-vaccinated primates were heavily infected following direct challenge, and the time to clearance was not different compared with naïve animals. Similarly, there was no difference in the kinetics or peak level of colonization between aP-vaccinated and naïve animals that were infected by natural transmission. Importantly, we also show in two experiments that aP-vaccinated animals transmitted B. pertussis to naïve cage mates. Together these data form the key finding of this study: aP vaccines do not prevent infection or transmission of Bordetella pertussis even 1 mo after completing the primary vaccination series. (source: http://www.pnas.org/content/111/2/787.long)

Whooping Cough Vaccine Failure

The FDA put out this press release about the vaccine’s failure:

There are two types of pertussis vaccines, whole-cell and acellular. Whole-cell pertussis vaccines contain a whole-cell preparation, which means they contain killed, but complete, B. pertussis bacteria. The acellular pertussis vaccine is more purified and uses only selected portions of the pertussis bacteria to stimulate an immune response in an individual. In response to concerns about the side effects of the whole cell pertussis vaccine, acellular vaccines were developed and replaced the use of whole-cell pertussis vaccines in the U.S. and other countries in the 1990s; however, whole-cell pertussis vaccines are still used in many other countries.
The FDA conducted the study in baboons, an animal model that closely reproduces the way whooping cough affects people. The scientists vaccinated two groups of baboons – one group with a whole-cell pertussis vaccine and the other group with an acellular pertussis vaccine currently used in the U. S. The animals were vaccinated at ages two, four, and six months, simulating the infant immunization schedule. The results of the FDA study found that both types of vaccines generated robust antibody responses in the animals, and none of the vaccinated animals developed outward signs of pertussis disease after being exposed to B. pertussis. However, there were differences in other aspects of the immune response. Animals that received an acellular pertussis vaccine had the bacteria in their airways for up to six weeks and were able to spread the infection to unvaccinated animals. In contrast, animals that received whole-cell vaccine cleared the bacteria within three weeks.
This research suggests that although individuals immunized with an acellular pertussis vaccine may be protected from disease, they may still become infected with the bacteria without always getting sick and are able to spread infection to others, including young infants who are susceptible to pertussis disease.

This article by Scientific American explains how the study was done:

To test their hypothesis, Merkel’s team members infected baboons with pertussis. Some of the animals had been vaccinated, and some had acquired natural immunity from a past bout of the illness. None of the vaccinated or naturally immune baboons fell ill, but the bacterium lingered for 35 days in the throats of the baboons that had received the acellular vaccine.

 

Shedding of the Whooping Cough Vaccine??

Regarding its ability to shed to others, it does not, since it is not a live virus vaccine. This section is quoted from the full length FDA Baboon study (here):

To test our hypothesis, infant baboons were vaccinated at 2, 4, and 6 mo of age with aP or whole-cell pertussis (wP) vaccines and challenged with B. pertussis at 7 mo. Infection was followed by quantifying colonization in nasopharyngeal washes and monitoring leukocytosis and symptoms. Baboons vaccinated with aP were protected from severe pertussis-associated symptoms but not from colonization, did not clear the infection faster than naïve animals, and readily transmitted B. pertussis to unvaccinated contacts. Vaccination with wP induced a more rapid clearance compared with naïve and aP-vaccinated animals. By comparison, previously infected animals were not colonized upon secondary infection.

Acellular Vaccines Fail to Prevent Infection Following Natural Transmission. Similar to our previous findings (18), all animals became colonized 7–10 d after cohousing with the infected animal.

If you are in a debate about this vaccine’s ability to protect, the following links are helpful:


Resources:

Recently Vaccinated Kids Are Spreading Pertussis (overview) by Barbara Loe Fischer,  NVIC

DTaP, TDap and Td Studies and articles

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