The CDC said it showed the jabs were “highly effective” even in settings where people are more likely to be exposed to the virus because of their occupations.
The 90 per cent figure was calculated by the number of infections found in the study group two weeks after they received two doses, at which point people are considered “fully vaccinated”, because that’s how long the body takes to produce antibodies. The jabs were also shown to reduce the risk of infection by 80 per cent two weeks after just one dose.
The study, among 3,950 participants across six states and over 13 weeks from December 14, was designed to identify both symptomatic and asymptomatic cases – participants submitted nasal swabs every week for testing, regardless of illness. As such, the study showed that the two mRNA vaccines reduced the risk of all infections, not just symptomatic infections, which is important for helping to prevent the spread of the disease.
There were 161 Covid infections among the unvaccinated workers in the study, compared with 16 in people who had only had one dose. There were just three infections in people who had received both doses of the vaccine. Three-quarters of the people included in the study got at least one jab during the period.
CDC director Dr Rochelle Walensky said: “This study shows that our national vaccination efforts are working. The authorized mRNA Covid-19 vaccines provided early, substantial real-world protection against infection for our nation’s health care personnel, first responders, and other frontline essential workers.”
She said the findings offered “hope” for those getting vaccinated.
“The authorized vaccines are the key tool that will help bring an end to this devastating pandemic,” she added.
Scientists said the results also suggested that the vaccines blocked transmission, although the study was not designed to test that.
Dr Angela Rasmussen, a virologist at the Georgetown Center for Global Health Science and Security in Washington, tweeted: “The mRNA vaccines are highly protective against Covid-19 disease and also against SARS-CoV-2 infection. This means that they are likely to significantly reduce transmission at population scale. Get vaccinated!”