The United States is experiencing a significant increase in whooping cough (pertussis) cases, with the current count being five times higher than at the same period last year, according to preliminary data from the US Centers for Disease Control and Prevention (CDC). This marks the highest incidence of infections since 2014, and there are no signs of a slowdown. The surge is particularly concerning as it indicates a return to pre-pandemic levels of the disease, which had been on the rise even before COVID-19 due to the waning effectiveness of current vaccines after two to three years.

Dr. Susan Hariri from the CDC’s National Center for Immunization and Respiratory Diseases presented these alarming figures at a recent expert meeting advising the US Food and Drug Administration (FDA) on vaccine decisions. The discussion focused on the potential use of human challenge trials as a method for testing new pertussis vaccines. These trials involve intentionally exposing vaccinated participants to the bacteria to evaluate the vaccine's effectiveness.

The decrease in whooping cough cases during the COVID-19 pandemic, attributed to reduced social interactions and preventive measures against the coronavirus, has reversed, with cases rising to 14,569 this year compared to 2,844 during the same period last year. The early symptoms of whooping cough are similar to a common cold, but the infection can progress to severe, full-body coughing fits that can cause vomiting or rib fractures and are characterized by a distinctive whooping sound as individuals gasp for air.

Antibiotics can treat whooping cough if caught early, but once the severe coughing stages commence, the only recourse is symptomatic relief and rest. The most vulnerable to severe infection and rare fatalities are unvaccinated infants and adults. However, there has been an increase in breakthrough infections among vaccinated individuals as well.

Pertussis vaccination rates in the US are high, around 90%, and did not drop during the pandemic. Despite this, the upward trend in infections persists. The cause of this increase is not fully understood, but it may be related to changes in the bacteria causing the infection. In the 1990s, the US shifted from a whole-cell vaccine that caused more side effects to a newer vaccine that offered less durable protection. This shift has corresponded with an increase in cases in other countries that adopted the newer vaccines.

The current vaccines' incomplete protection allows the infection to spread and has led to bacterial mutations that evade the vaccine-induced immune response. As a result, the protection provided to young children wanes quickly, often before the recommended booster at age 11. Infections are rising among older children and teens as their protection diminishes, prompting the search for more effective and longer-lasting vaccines.

The challenge in developing new vaccines is the cyclical nature of whooping cough, with major outbreaks every three to five years, making it difficult to predict and plan testing. Human challenge studies could provide a controlled environment to test new vaccines, but there are concerns about whether these studies can accurately measure the vaccines' ability to prevent severe disease and if they can provide sufficient evidence for vaccine approval.

The FDA advisory committee discussed models presented by labs in Canada and the UK for conducting pertussis challenge trials. The experts had mixed reactions, with some expressing excitement about the potential speed of vaccine development using this approach, while others questioned whether the studies were refined enough to yield reliable results. The debate reflects the urgency of finding a more effective vaccine against whooping cough amid a significant public health challenge.