Pertussis, more commonly known as whooping cough, is a highly contagious bacterial infection of the upper and lower airway that is terrible in adults and especially nasty in young children. Infants in particular are at increased risk of severe disease. Roughly 20% of infants who develop pertussis will require hospitalization, and one out of every hundred hospitalized infants will die from either the respiratory complications or severe inflammation of the brain that can sometimes occur.
Though pertussis is not as common as it was prior to the development and widespread uptake of an effective whole-cell vaccine in the 1940s, when it was diagnosed around 200,000 each year in the United States, it is still around. Cases actually began to increase from a nadir of 1,000 reported in 1976 after increasing numbers of parents began to refuse the vaccine because of rumors, anecdotes, and anti-vaccine propaganda involving permanently injured children. Another factor in the resurgence of pertussis was the switch from the whole-cell vaccine to a less effective acellular vaccine for infants in 1991 out of an abundance of caution.
Multiple studies have since found that the whole-cell vaccine did not cause permanent brain injury in children. It was associated, however, with an increased risk of febrile seizures and, very rarely at less than 1 per 100,000 doses, an acute encephalopathy. Its appears that we are going to be stuck with the inferior vaccine for the foreseeable future.
Protective antibodies produced after completing the childhood acellular pertussis vaccine series wane over time, resulting in the need for a booster at age 11 or 12 and then every ten years after that. This means, unfortunately, that many, many people are out there right now with less than ideal immunity to pertussis, and tetanus as well since it is typically boosted along with pertussis. This is bad for healthy adults, worse for any immune compromised adults or adults with health conditions that put them at higher risk of severe disease, but really bad for babies.
The primary vaccine series begins at 2 months of age, leaving the very young at risk of disease in general, and more severe disease to boot. Most deaths and hospitalizations have naturally occurred in infants during the first 3 months of life. If a mother has circulating antibodies during pregnancy, these can be passed to the baby and provide some protection until they begin receiving their vaccinations. But again, naturally, this can’t occur if a mother has no pertussis antibodies to share.
In an effort to reduce the incidence of pertussis in this population, the American Committee on Immunization Practices (ACIP) began recommending “cocooning” in 2006. This is when unvaccinated family members, caregivers, and mothers who have just delivered receive a pertussis vaccine so that there is a protective barrier of immunity surrounding the baby. But this recommendation didn’t go far enough.
In 2011, ACIP recommended that all unvaccinated pregnant people receive their first dose of pertussis vaccine during pregnancy in order to allow for protective maternal antibodies to cross over to the baby prior to delivery. The evidence at that time was abundantly clear that the vaccine was safe for both mothers and the developing fetus. In 2013, this recommendations was again improved by encouraging a dose of Tdap with each pregnancy even in a previously vaccinated mother.
First, some good news.
A study published in JAMA Pediatrics this month looked at cases of pertussis diagnosed in infants in the United States between 2000 and 2019 to determine how effective, if at all, recommendations to immunize pregnant people were in reducing the incidence. During this time period, more than 57,000 cases were reported in infants, with a third being diagnosed in babies less than 2 months of age. During the pre-recommendation period, the average yearly incidence of pertussis was 165 per 100,000 (205/100,000 in 2012!) of these younger infants compared to 19.7 per 100,000 in the comparison group of infants between 6 and 12 months.
In the post-recommendation period, the incidence of pertussis in the younger infant group decreased substantially to an average of about 80 per 100,000. The incidence of pertussis in the comparison group did not change in the post-recommendation period, which isn’t surprising. The point of maternal vaccination is to reduce risk during the period before the primary childhood vaccination series begins at 2 months.
Now, a bit of not so good news.
When the recommendation to vaccinate all pregnant people came out, the uptake increased steadily. In 2014, about 25% got the vaccine. By 2016, that number had increased to roughly 50%. But in 2019, it had only increased another 5%, still leaving millions of newborns unprotected. This strategy obviously works better as more pregnant people get the vaccine, so we still have a lot of work to do.