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High COVID-19
Vaccine Efficacy Estimates From Clinical Trials Holding Up Under Field
Conditions
Initial estimates of vaccine efficacy from clinical
trials of the Pfizer and Moderna mRNA vaccines were surprisingly high
at 94-95% efficacy in preventing symptomatic COVID-19. Now
reports from studies of vaccine efficacy under real world conditions
have proven almost equally high.
Mayo Study
Pawlowski and colleagues at the Mayo clinic carried out
a retrospective analysis of 31, 069 health care personnel and
residents of long term care facilities who were vaccinated between
December and February with at least one dose of either the Pfizer or
Moderna vaccines. Their experience was compared to that of
unvaccinated matched controls.
As reported on the preprint server medRxiv,
administration of two doses among persons who were at risk for at
least 36 days after the first dose was 88.7% effective in preventing
infection. Also, vaccinated persons who subsequently did get
ill had significantly lower 14 day hospital admission rates than
controls (3.7% vs 9.2%).
Because the participants in this observational study
were health care workers and residents of long term care facilities,
and because participants knew they were vaccinated, their risk of
exposure was likely higher than that of participants in the original
clinical trials and therefore the estimates in this study are likely
to be an underestimate of the true efficacy, according to Pawlowski
and co-authors.
The authors conclude that the estimates of vaccine
efficacy in this real world population are “on par” with those
observed in the clinical trial populations. What is new in this report
is that the vaccines were effective among persons who could be
expected to be at highest risk of acquiring COVID-19 and the vaccines
were effective not only in reducing symptomatic illness but also in
reducing infection as measured by a single PCR test.
Israeli Study
In another post-trial observational study in Israel,
the times of Israel reported that only 608 persons out of a total of
602,000 fully vaccinated persons have been infected and none have
died. Only 21 of these have been hospitalized and only 7 with severe
symptoms. According to the investigator,
Ekka Zohar
“The data proves beyond a shadow of a doubt that this is the most
effective way to defeat the pandemic.” Using an equivalent sample of
unvaccinated controls, Zohar said vaccine efficacy is very close to
that reported from the clinical trials.
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