Could we Vaccinate Twice as Many People?

Michael Minna raises an intriguing possibility in an opinion piece in the New York Times: Instead of giving people two shots of a vaccine, we could give them only one and delay the second dose until enough vaccines are available.

The current vaccines are designed to be administered in two steps. The first step provides the initial exposure of the body to virus antigens. The second is a booster designed to increase and prolong immunity. Here is the chart from the Pfizer/BioNTech vaccine paper published in the New England Journal of Medicine:


We can see that the curves start to diverge at around day twelve – nine days before the second shot was administered. It is plausible that the immune reaction will be enhanced by a second shot. It is, however, somewhat unclear, by how much. Based on the publish data it looks like vaccine effectiveness after the first shot is at least between 50-80%, but that is hard to tell as most participants have received two shots. Some of the trial participants dropped out of the study before receiving a second shot – these would be the obvious starting point to look for red flags of missing immunity.

Many vaccines are administered in a two-shot regime, but sometimes these shots are administered years apart (for example the Measles-Mumps-Rubella vaccine). This will likely not be the case for the Covid-19 vaccine, but it is also not clear whether we need a second vaccine after 21 days (or 28 in the Moderna case). The central question therefore is whether that second shot could be better used by giving it to someone else who has received no protection at all.

What follows from this? We should immediately start looking for evidence whether or not a single vaccine shot grants decent protection (e.g. the 50% threshold demanded by the FDA). That means we should look at the existing data, as well as try to start new trials as soon as possible. There are still thousands of volunteers who would be willing to participate in such a trial. Even without further trials (and without a deeper look from my side) I would say the existing data quite strongly suggests that we should be much more concerned with getting the first shot out to as many as possible than about administering the booster to those already vaccinated once.

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