Can We Do Twice as Many Vaccinations as We Thought?

It’s been a very good month for Covid-19 vaccines. Last week, the Food and Drug Administration provided an emergency authorization for a vaccine produced by Pfizer-BioNTech. On Thursday, an advisory committee recommended authorizing a vaccine by Moderna, and the F.D.A. is expected to authorize it soon.

These vaccines are a triumph. In large-scale trials with tens of thousands of participants, both demonstrated a stunning 95 percent efficacy in preventing Covid-19 — a number exceeding our best hopes.

Both vaccines are supposed to be administered in two doses, a prime and a booster, 21 days apart for Pfizer and 28 days for Moderna. However, in data provided to the F.D.A., there are clues for a tantalizing possibility: that even a single dose may provide significant levels of protection against the disease.

If that’s shown to be the case, this would be a game changer, allowing us to vaccinate up to twice the number of people and greatly alleviating the suffering not just in the United States, but also in countries where vaccine shortages may take years to resolve.

But to get there — to test this possibility — we must act fast and must quickly acquire more data.

For both vaccines, the sharp drop in disease in the vaccinated group started about 10 to 14 days after the first dose, before receiving the second. Moderna reported the initial dose to be 92.1 percent efficacious in preventing Covid-19 starting two weeks after the initial shot, when the immune system effects from the vaccine kick in, before the second injection on the 28th day

That raises the question of whether we should already be administrating only a single dose. But while the data is suggestive, it is also limited; important questions remain, and approval would require high standards and more trials.

First, the science. While the vaccine trials were designed to evaluate a two-dose regimen, some immunity might be acquired before a second dose is administered. We know, for instance, that a Covid-19 infection appears to yield protection for at least six months. While infections are not vaccinations, and while we need more data on this, it’s plausible that the immunity gained from a vaccination may turn out to be even stronger than what comes from an infection. The reason we do a second — booster — vaccination is that these later doses help to solidify immune memory, in part by giving extra training to the cells that produce antibodies, a process called affinity maturation. But this process begins with the single dose, and the evidence collected between the time of the first and second doses in tens of thousands of people in the Phase 3 trials suggests that the level of affinity maturation may provide enough protection to meet the standards we have set for vaccine approval during this pandemic even without the second dose.

While we know that the single dose can protect against disease, we don’t yet know how long this immune protection will last, and at what level. However, there is no rule that says that vaccines must be boosted within weeks of each other. For measles, the booster dose is given years after the first dose. If the booster dose could be given six months or a year after the first dose, while maintaining high efficacy before the second dose, that would allow twice as many people to get vaccinated between now and later next year, accelerating herd immunity — greatly helping end the crisis phase of the pandemic in the United States.

by Zeynep Tufekci and Michael Mina, NY Times | Read more:
Image: Michael A. McCoy for The New York Times