More boosters on the way?
In the second half of the meeting, the CDC also signaled that it was considering the use of boosters for people whose immunity might have waned in the months since they had completed their vaccine series, particularly seniors. About 75% of people hospitalized with vaccine breakthrough cases are over age 65, according to CDC data.
Those considerations are becoming more urgent as the Delta variant continues to pummel less vaccinated states and counties.
In its presentation to the ACIP, Heather Scobie, PhD, MPH, a member of the CDC’s COVID Response Team, highlighted data from Canada, Israel, Qatar, and the United Kingdom showing that, while the Pfizer vaccine was still highly effective at preventing hospitalizations and death, it’s far less likely when faced with Delta to prevent an infection that causes symptoms.
In Israel, Pfizer’s vaccine prevented symptoms an average of 41% of the time. In Qatar, which is also using the Moderna vaccine, Pfizer’s prevented symptomatic infections with Delta about 54% of the time compared with 85% with Moderna’s.
Dr. Scobie noted that Pfizer’s waning efficacy may have something to do with the fact that it uses a lower dosage than Moderna’s. Pfizer’s recommended dosing interval is also shorter – 3 weeks compared with 4 weeks for Moderna’s. Stretching the time between shots has been shown to boost vaccine effectiveness, she said.
New data from the Mayo clinic, published ahead of peer review, also suggest that Pfizer’s protection may be fading more quickly than Moderna’s.
In February, both shots were nearly 100% effective at preventing the SARS-CoV-2 infection, but by July, against Delta, Pfizer’s efficacy had dropped to somewhere between 13% and 62%, while Moderna’s was still effective at preventing infection between 58% and 87% of the time.
In July, Pfizer’s was between 24% and 94% effective at preventing hospitalization with a COVID-19 infection and Moderna’s was between 33% and 96% effective at preventing hospitalization.
While that may sound like cause for concern, Dr. Scobie noted that, as of August 2, severe COVD-19 outcomes after vaccination are still very rare. Among 164 million fully vaccinated people in the United States there have been about 7,000 hospitalizations and 1,500 deaths; nearly three out of four of these have been in people over the age of 65.
The ACIP will next meet on August 24 to focus solely on the COVID-19 vaccines.
A version of this article first appeared on Medscape.com.