Effectiveness of Acellular Pertussis Vaccine Wanes Over Time

By Will Boggs MD

June 14, 2019

NEW YORK (Reuters Health) - The risk of acquiring pertussis is significantly higher in undervaccinated and unvaccinated children, but even among fully vaccinated children, it increases over the course of several years, researchers report.

"In this study, we found although children who were unvaccinated had a 13 times increased risk of pertussis, the vast majority (82%) of pertussis cases were age-appropriately vaccinated," said Dr. Ousseny X. Zerbo from Kaiser Permanente Northern California (KPNC), in Oakland.

"Only 18% were either unvaccinated or undervaccinated. The finding that most pertussis cases occurred among highly vaccinated children suggests that waning of vaccine immunity played a strong role in recent pertussis cases," he told Reuters Health by email.

A number of countries, including the U.S., have seen large pertussis outbreaks since the replacement of the whole-cell pertussis vaccine by an acellular vaccine in the 1990s. Vaccine coverage in the U.S. remains about 95%, suggesting that waning of vaccine immunity may play a strong role in these outbreaks.

Dr. Zerbo's team used data from nearly 470,000 children, followed for an average 4.6 years from birth, to determine pertussis risk by vaccination status and time since last dose of acellular pertussis vaccine.

During follow-up, there were 738 PCR-confirmed pertussis cases: 70% among fully vaccinated children, 12% among fully vaccinated plus one dose, 5% among undervaccinated children, and only 13% among unvaccinated children, the team reported in Pediatrics, online June 10.

After adjustment for other factors, pertussis risk among unvaccinated children was 13.53 times higher and among undervaccinated children 86% higher than among fully vaccinated children across all follow-up and age groups. Children who were fully vaccinated plus one dose were 52% less likely than fully vaccinated children to develop pertussis.

Among children 19 to 83 months old, the risk of pertussis was five times higher for those whose last vaccine dose was three or more years earlier compared with those whose last dose was less than one year earlier.

Similarly, among older children, the risk of pertussis was more than doubled six or more years after the last vaccination compared with less than three years.

"This study is the first to look at the role of both undervaccination and waning of vaccine immunity in the occurrence of pertussis, and we found that both matter," Dr. Zerbo said. "Although pertussis vaccine immunity wanes, it is important that children continue to be vaccinated according to Centers for Disease Control and Prevention (CDC)'s vaccination recommendation. The pertussis vaccine is effective at preventing pertussis but for a short period of time. If a child is not vaccinated or is undervaccinated his/her risk is much higher than a child who is fully vaccinated."

"Physicians should continue offering and encouraging parents to vaccinate their children until a better vaccine is available," he said. "Unvaccinated children have a very high risk of getting pertussis and the best way to prevent the disease is to give all recommended doses of pertussis vaccines."

In an editorial, Dr. Kathryn M. Edwards from Vanderbilt University, in Nashville, Tennessee, writes, "First, given the markedly increased risk of pertussis in unvaccinated and undervaccinated children, universal DTaP vaccination should be strongly recommended. Second, the addition of maternal Tdap vaccination administered during pregnancy has been shown to significantly reduce infant disease before primary immunization and should remain the standard. How to address the waning immunity of DTaP vaccination is more problematic."

"Studies are ongoing to determine if adjuvants can be added to acellular vaccines to modify their T-cell responses to a more protective immune response or whether the T-cell response remains fixed once primed with DTaP vaccine," she adds. "In the interim, investigators at KPNC, other large health delivery organizations, and the Centers for Disease Control and Prevention need to continue to monitor the pertussis burden in the United States and work with scientists to improve the existing pertussis vaccines."

Dr. Stanley Plotkin, emeritus professor of pediatrics at the University of Pennsylvania, in Philadelphia, now at Vaxconsult, in Doylestown, Pennsylvania, has written about the problem of waning effectiveness of pertussis vaccines.

"These data are confirmatory of others," he said. "The reason is that the acellular vaccine generates a T helper-cell type 2 response that does not give good B-cell memory. The old cellular vaccine was reactogenic, but generated a T helper-cell type 1 response that gave better B-cell memory and therefore longer efficacy."

"Efforts are being made to improve acellular vaccines in this regard," he told Reuters Health by email.

SOURCE: https://bit.ly/31tEicv and https://bit.ly/2KGwtdG

Pediatrics 2019.