A Half Dose of Moderna is More Effective Than a Full Dose of AstraZeneca

A Half Dose of Moderna is More Effective Than a Full Dose of AstraZeneca

Today we are releasing a new paper on dose-stretching co-authored by Witold Wiecek, Amrita Ahuja, Michael Kremer, Alexandre Simoes Gomes, Christopher M. Snyder, Brandon Joel Tan and myself.

The paper makes three big points. First, Khoury et al (2021) just published a paper in Nature which shows that “Neutralizing antibody levels are highly predictive of immune protection from symptomatic SARS-CoV-2 infection.” What that means is that there is a strong relationship between immunogenicity data that we can easily measure with a blood test and the efficacy rate that it takes hundreds of millions of dollars and many months of time to measure in a clinical trial. Thus, future vaccines may not have to go through lengthy clinical trials (which may even be made impossible as infections rates decline) but can instead rely on these correlates of immunity.

Here is where fractional dosing comes in. We supplement the key figure from Khoury et al.’s paper to show that fractional doses of the Moderna and Pfizer vaccines have neutralizing antibody levels on par with those of many approved vaccines. Indeed, a one-half or one-quarter dose of the Moderna or Pfizer vaccine looks to be more effective than the standard dose of some of the other vaccines like the AstraZeneca, J&J or Sinopharm vaccines. The point is not that these other vaccines aren’t good–they are great! The point is that by using fractional dosing we could rapidly and safely expand the number of effective doses of the Moderna and Pfizer vaccines.

Third, we show that under plausible scenarios it is better to start vaccination with a less efficacious vaccine than to wait for a more efficacious vaccine. Thus, Great Britain and Canada’s policies of starting First Doses first with the AstraZeneca vaccine and then moving to second doses, perhaps with the Moderna or Pfizer vaccines is a good strategy.

It is possible that new variants will reduce the efficacy rate of all vaccines indeed that is almost inevitable but that doesn’t mean that fractional dosing isn’t optimal nor that we shouldn’t adopt these policies now. What it means is that we should be testing and then adapting our strategy in light of new events like a battlefield commander. We might, for example, use fractional dosing in the young or for the second shot and reserve full doses for the elderly.

One more point worth mentioning. Dose stretching policies everywhere are especially beneficial for less-developed countries, many of which are at the back of the vaccine queue. If dose-stretching cuts the time to be vaccinated in half, for example, then that may mean cutting the time to vaccination from two months to one month in a developed country but cutting it from two years to one year in a country that is currently at the back of the queue.

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