It might not become clear which Covid-19 vaccines work best -- or whether they work at all -- until after they’ve already been authorized and are being given to many people, a team of experts said Tuesday.
With 44 vaccines in clinical trials -- meaning they are being tested in people -- it will be difficult to tell which ones are the most effective in preventing disease, said the experts led by Dr. Susanne Hodgson of Oxford University’s Jenner Institute.
People may expect the vaccines will at least protect against severe infection. “However, protection against severe disease and death is difficult to assess in phase 3 clinical trials due to the unfeasibly large numbers of participants required,” the experts wrote in a review in the journal Lancet Infectious Diseases.
“Instead, data to address this end point might be available only from large phase 4 trials or epidemiological studies done after widespread deployment of a vaccine.”
So called Phase 4 trials are usually studies done after regulators approve a vaccine.
Prevention of severe infections: Vaccine developers said at first, people may have to make do with a vaccine that prevents detectable infection, and hope that also means it is preventing severe infections.
“Only a small proportion of individuals infected with SARS-CoV-2 develop severe disease, which means an extremely large number of volunteers is needed in a clinical trial for there to be enough cases to get a reliable measure of vaccine efficacy,” study co-author Kate Emary, a researcher at the University of Oxford, said in a statement.
“This means that it is likely that we will only know if a vaccine protects against severe disease once it has been deployed and given to a large population,” she added.
Human challenge trials: The team is also urging caution in the use of data provided by controlled human infection studies or challenge trials now underway in Britain. “It is unclear if results from these studies, which are likely to only include young volunteers, will predict vaccine efficacy in older adults,” they wrote.
The researchers are urging vaccine makers to commit to a long-term follow-up of volunteers in Covid-19 vaccine trials because it’s “important, both to evaluate efficacy against severe disease and mortality, and to ensure ongoing evaluation of vaccine safety.”
Four vaccines under development in the US are in the most advanced, Phase 3 clinical trials.
