This article was first published in The Montreal Gazette.
In an effort to vaccinate as many people as quickly as possible, governments around the world have been openly debating how to stretch the vaccine supply as far as possible. The U.K. government has decided that it may swap one vaccine for another for the second dose, the United States is discussing whether people could receive two half doses of the Moderna vaccine rather than two full doses and in Canada the discussion is about delaying or deferring the second dose altogether.
The rationale for these decisions is not hard to understand. The sooner everyone can be vaccinated, the faster the pandemic will come to an end. But there is one fundamental and major shortcoming to all of these proposed strategies. They have not been tested.
The suggestion that vaccines might be interchangeable is not entirely without merit, given that the Pfizer and Moderna vaccines have the same basic mechanism of action. However, swapping vaccines has never been studied and no one knows whether it would be safe or effective. The issue is somewhat murkier when you introduce the AstraZeneca vaccine into the mix. This is a different type of vaccine, so its interchangeability with the others is even harder to guess at. Given the absence of data, substituting a different vaccine for the second dose would be a giant population-wide experiment that no one has actually volunteered for.
Delivering a half dose of the vaccine would be helpful in that it would allow twice as many people to be vaccinated, given the same manufacturing capacity. The justification comes from early studies showing that both lower and higher doses of the vaccine were able to produce antibodies in patients. But generating antibodies is not proof of immunity. The point of Phase 3 clinical trials is to test the effectiveness of vaccines (and all medications) against hard clinical endpoints. Measuring antibody levels provides helpful information, but antibody levels do not necessarily correlate with infection rates. Assuming that they do is simply an assumption.
Delaying or deferring the second dose of the Pfizer or Moderna vaccine has the same problem. Very few people in the Moderna study got only one dose of the vaccine. If you examine the data submitted to the U.S. Food and Drug Administration, in the roughly 1,000 people who got only a single dose of the Moderna vaccine, the vaccine efficacy was about 80 percent. But because of the relatively small number of people in this analysis, the margin of error means it could have been as high as 93 percent or as low as 55 percent. The Pfizer data is harder to interpret because there is no separate analysis of people who only got one dose. In the period between dose No. 1 and dose No. 2, vaccine efficacy was 52 percent (with the margin of error putting it between 30 percent and 68 percent).
You can make a calculated decision that some immunity in more people is better than more immunity in fewer people. I have my reservations, though, as the people not getting their second dose are largely health care workers and residents of long-term care facilities who are the highest risk group. If the delay in the second dose is simply a matter of one or two weeks, then the consequences will probably be minimal. But longer delays may mean that immunity wanes and people will need to get re-vaccinated. The truth is, we simply don’t know.
We went to the trouble and expense of conducting large randomized trials with tens of thousands of people to prove with high degrees of certainty that these vaccines work. It seems counterproductive to ignore the results.