Rising instances of whooping cough prompt questions about utilizing infection for improving an advanced vaccine.
It's experiencing the highest number of pertussis infections since 2014, with no signs of decreasing, as mentioned by Dr. Susan Hariri from the CDC's National Center for Immunization and Respiratory Diseases during a meeting with FDA advisors last Friday.
This year's increase in cases has returned pertussis infection levels to pre-pandemic figures. The US was already seeing an upward trend in pertussis cases before the Covid-19 pandemic, attributed to the fact that the current vaccines provide strong initial protection that decreases after two to three years.
Dr. Archana Chatterjee, a pediatric disease specialist and dean of the Chicago Medical School, stated during the meeting that pertussis vaccines have been effective for many decades but are no longer as effective as they once were. She is a member of the FDA's Vaccines and Related Biological Products Advisory Committee.
On Friday, the committee gathered to discuss the possibility of allowing pharmaceutical companies to voluntarily expose individuals to pertussis to test their vaccines, a practice known as a human challenge trial.
Pertussis cases on the rise for decades
Pertussis cases dropped significantly during the pandemic as people limited social interaction and implemented measures to protect against Covid-19.
The US recorded approximately 6,000 pertussis cases in 2020, which further decreased to around 2,000 cases in 2021. The number of cases increased to about 3,000 in 2022 and around 5,000 in the previous year.
The US has recorded 14,569 cases this year, compared to 2,844 at the same time last year, according to the CDC's Nationally Notifiable Diseases Surveillance System.
Symptoms of pertussis begin with a runny nose, sneezing, a mild fever, and a tickly cough, which gradually develops into a severe coughing fit. The force of the coughing fits can cause patients to vomit or break ribs and is often accompanied by a whooping sound as the person tries to catch their breath. Antibiotics can treat the infection if caught within the first few weeks, but only if administered before the onset of the debilitating, painful cough.
Unvaccinated infants and adults are most prone to severe infection and, in rare cases, death. However, doctors have observed vaccinated individuals experiencing infections as well.
Children typically receive five doses of the pertussis vaccine – DTaP or TdaP – by the age of six. Boosters are recommended for children aged 11 or 12. Pregnant women are also advised to receive the vaccine, and adults are suggested to get boosters every 10 years.
Pertussis vaccination rates in the US are high, with an estimated 90%. According to Hariri, vaccination rates did not decrease during the pandemic, as they did in some other countries.
Despite the high vaccination rates, the upward trend in cases has continued.
Vaccine protection wanes
The cause for the increase in cases is not fully understood. Dr. Tod Merkel of the FDA's Office of Vaccines Research and Review suggested during a presentation to the committee that the changes in the bacteria causing the infection could be contributing to the rise in cases.
In the 1990s, the US shifted to a newer type of pertussis vaccine that caused fewer side effects but offered less protection with shorter-lasting effects compared to the older whole-cell vaccine. This incomplete protection has allowed the infection to persist and caused the bacteria to evolve to evade the immune response generated by the vaccine.
Merkel reported that 90% of bacterial strains associated with infections in the US are now linked to mutations that bypass the protection provided by the vaccine.
Young children vaccinated against pertussis are experiencing a rapid waning of protection, sometimes within two to three years – well before the recommended booster at age 11. As a result, older children and teens are increasingly falling victim to infections as their protection wears off.
The hunt is on for improved, long-lasting vaccines against whooping cough with fewer side effects. However, testing these new vaccines has been challenging due to the cyclical nature of whooping cough outbreaks, which are difficult to predict. Enter human challenge studies.
Human challenge studies, which expose volunteers to the infection to test the effectiveness of new vaccines, may provide a potential solution to this testing challenge. However, the FDA and other regulatory bodies will need to carefully consider the ethical implications and potential risks of such studies before they can be implemented.
While many individuals exhibit minimal symptoms during the initial stages, investigators have challenges in assessing the vaccine's efficacy against mild early symptoms and its capacity to impede bacterial colonization in nasal passages. Pertussis challenge models, unfortunately, fail to offer direct insights into the vaccine's potential to deter severe disease, which is typically the primary objective of vaccines.
Additionally, medical professionals are unaware of the specific immune mechanisms that offer protection against pertussis, hindering their ability to evaluate if the immunity derived from these experimental vaccines matches or even surpasses that granted by approved vaccines - a strategy known as bridging.
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On a Friday, labs in Canada and the UK unveiled their strategies for conducting pertussis challenge trials.
Subsequently, the FDA advisors were questioned about their opinions on whether these trials could accurately assess the essential aspects and if such results could serve as the groundwork for a vaccine approval.
The proposed strategies by the labs generated a variety of perspectives from the experts. On the positive side, they agreed that this method had potential to expedite the testing of new vaccines. Nonetheless, they harbored doubts about the adequacy of these studies' refinement.
As Dr. Melinda Wharton, the associate director for vaccine policy and clinical partnerships at the CDC’s National Center for Immunization and Respiratory Diseases, expressed, "I am highly inclined towards this, but I am not entirely convinced if we're at that point yet."
The ongoing rise in pertussis cases poses a significant threat to public health. Despite high vaccination rates, the protection offered by pertussis vaccines seems to wane over time, making individuals more susceptible to infection.
