COVID-19 'challenge trials' can help scientists better understand the immune system's response to the virus, which could yield improved vaccines and treatments
In 2016, for instance, to test if a single-dose oral vaccine called Vaxchora prevented a cholera infection in healthy adults, U.S. scientists exposed study participants to the cholera bacteria either 10 days or three months after vaccinating them. Theshowed that Vaxchora reduced the likelihood of severe diarrhea in vaccinated individuals by between 80 and 90 percent. Because cholera is a rare disease in the United States, “you can’t really do a big field trial,” says Neuzil.
Human challenge trials, albeit with their obvious risks, may be better suited to understand how long a previous COVID-19 infection or vaccine shot provides immunity and if indicators other than antibodies, which are currently used to assess the efficacy of vaccines and other treatments, may be more relevant to measure success, saysResults from conducting similar experiments in animals may not accurately reflect human responses to a disease or treatments.
“With a challenge study you can do that because you can get [blood, tissue, respiratory] samples before infection and continue to sample following the infection,” Neuzil says, as the patients remain under observation in the controlled environment of a hospital while they continue to shed the virus and when they return for checkups over following months. Here, asymptomatic infections are also identified.
However, obtaining clearances and laying the groundwork for such studies can take months, but “it becomes particularly tricky for coronaviruses and influenza viruses that keep changing,” Neuzil says. By the time an experiment reveals a safe and effective viral dose to expose challenge study participants to, another variant comes along and requires researchers to determine a new dose for challenge trials that might involve the new variant.
But because such studies don’t require hundreds of thousands of participants as traditional clinical trials do to work, they could
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