by Raywat Deonandan, PhD
Epidemiologist & Associate Professor
University of Ottawa
(I add my credentials to these COVID-19 blog posts in case they get shared. I want readers to know that my opinion is supposedly an educated and informed one)

Earlier this year, I wrote a post explaining how prior to the rise of the Omicron variant, COVID-19 mRNA vaccines were actually not bad at preventing both infection and transmission of the disease. Hence the strong evidence base for policy measures like vaccine mandates at work and vaccine passports at places like restaurants.

Omicron changed all that to a high degree by greatly reducing the vaccines’ ability to curtail infection and transmission, but not their ability to provide strong protection against the worst measurable COVID outcomes, like hospitalization and death.

That analysis was based on the assumption that most people had received only two doses of the vaccine. But I ended that post by citing some evidence showing that a third dose restored much of the jab’s effectiveness at preventing infection.

As I write this, that original Omicron strain has been supplanted by more transmissible subvariants. Currently, the BA.4 and BA.5 subvariants account for the majority of cases in North America. Both subvariants show an ability to overcome immunity garnered from both vaccination and prior infection from Omicron, suggesting that even three doses are no longer effective in preventing infection and transmission.

We are now in the era of 4th doses. So I thought it would be helpful to revisit the evidence around whether four doses can help prevent infection.

Spoiler alert: the evidence, as it is, suggests that four doses definitely increases our ability to avoid infection altogether. But there is debate about exactly how much that increase is, and how long it will last.

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