A crucial question: How well do the vaccines prevent asymptomatic infections?
When federal health authorities authorized the first Covid-19 vaccines late last year, they acted based on evidence proving that vaccinated people were far less likely than unvaccinated people to develop Covid-19 symptoms.
But as public health officials emphasized at the time, there wasn't enough evidence to determine whether the vaccines also prevented people from developing asymptomatic coronavirus infections. That matters because, at least in theory, a vaccinated person with an asymptomatic infection could still transmit the coronavirus to others.
To gauge how likely post-vaccination transmission is in the real world, researchers are conducting mass coronavirus screenings of vaccinated people, hoping to detect any asymptomatic cases.
For instance, according to
Vox, a working
paper—not yet peer-reviewed—released Friday in
The Lancet assessed thousands of Covid-19 screenings among health care workers at a hospital in Cambridge, England, including both unvaccinated staff and staff who had received the Pfizer/BioNTech vaccine.
The researchers found that, among the unvaccinated staff, 0.80% tested positive for the coronavirus. In comparison, among staff who had received the vaccine, only 0.37% tested positive less than 12 days after their vaccination, and just 0.20% tested positive more than 12 days after vaccination.
According to Mike Weekes, an infectious disease specialist at
Cambridge University and co-leader of the study, the results suggest the risk of developing asymptomatic Covid-19 is four times less among health care workers who have been vaccinated for at least 12 days.
Meanwhile, a press released on a pre-published, not-yet-peer-reviewed paper from the Israeli Health Ministry and Pfizer found that the vaccine appeared to reduce all coronavirus infections—including asymptomatic infections—by 89.4% and symptomatic infections by 93.7%.
While both studies focused specifically on Pfizer's vaccine, experts told
Vox's Kelsey Piper that Moderna's vaccine would likely produce similar results, since both vaccines work similarly.
For its part, Moderna found in its supplemental
research submitted to
FDA—based on nasal swab test data—that only 14 of the 14,134 people given its vaccine had an asymptomatic case of Covid-19, compared with 38 of the 14,073 people in the control group.
