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The first Covid-19 vaccines from Pfizer and BioNTech are distributed in the UK this week, marking a turning point in the pandemic. As more countries move to license and disseminate vaccines, the end of the pandemic may feel tantalizingly near. But before we can get a clear idea of when and how life could return to normal, we have a number of pending questions to answer.
One of the remaining unknowns: once an individual is vaccinated, how long will their immunity to Covid-19 last? To answer this question and others, scientists must collect more data from vaccine trials, some of which will simply take longer to collect.
At the moment, only three candidate vaccines – those from Pfizer and BioNTech, Moderna and AstraZeneca, and the University of Oxford – have published significant safety and efficacy data from their late-stage clinical trials. These data are sufficient for global regulatory bodies to assess whether or not to authorize vaccines for widespread use. But all of the Covid-19 vaccines in the works will continue those trials well beyond authorization.
The question of how long a vaccine will work “will be part of ongoing studies,” said Albert Bourla, Pfizer’s chief executive, in a press call with the International Federation of Pharmaceutical Manufacturers and Associations on Dec. 8. . The answer does not simply depend on the performance of a vaccine; It could also change depending on how SARS-CoV-2 works in the real world.
Preliminary evidence from individual cases has suggested that it is possible to develop Covid-19 more than once. Scientists are still unclear why this happens; In theory, after we get sick, our immune cells should develop antibody defenses that will be ready to deal with future infections even faster. It could be that Covid-19 is more like seasonal viruses, such as the cold or flu, for which we do not retain long-term immunity. In that case, combat the pandemic and agreement would require repeat vaccinations.
Cold and flu viruses also mutate substantially year after year, so we need yearly flu shots; there is no vaccine for the viruses that cause the common cold. In that case, we may have to redesign the Covid-19 vaccine before taking it again. But so far, it appears that SARS-CoV-2 is spreading faster than it mutates, which means we probably don’t need new vaccines every year.
It could also be that reinfections simply occur when people don’t get sick enough to develop strong antibody responses against the virus. If that’s the case, there is a chance that vaccines could elicit a stronger immune response than these weaker cases, protecting people for several years. The first mouse studies of SARS-CoV-2-targeted mRNA vaccines provided immunity to mice 13 weeks after they received two doses, which could translate into years of time in humans.
But the fact is, no one really knows the answers to these questions. Scientists will only have to keep gathering information from ongoing vaccine trials to be sure.
The Pfizer / BioNTech and other trials will span dozens of months. Tens of thousands of trial participants will continue to communicate regularly with healthcare providers, providing more data on long-term safety and efficacy, and receiving supportive care should they need it. If participants who received the vaccine develop Covid-19 after a year, for example, scientists will know that annual boosters may be necessary.
These long-term follow-ups may also answer other outstanding questions about Covid-19 vaccines. They can assess whether vaccine candidates actually stop viral transmission or are only protecting people from getting sick. (Theoretically, vaccines should stop transmission as they eliminate the possibility of the virus replicating within us). They will also be able to demonstrate any long-term side effects of these vaccines.
As this information becomes available, it can help inform public health decisions about reopening schools, restaurants, or unrestricted travel. But until we have a complete understanding of both the virus and the vaccines that protect against it, basic public health tools – tracking local transmission and hospitalization rates – are one of our best weapons.
