Covid-19 vaccines from Pfizer Inc. and Moderna Inc. effectively prevented coronavirus infections, not just illness, with substantial protection evident two weeks after the first dose, government researchers said.
Two doses of the vaccines provide as much as 90% protection against infection, according to data from US Centers for Disease Control and Prevention study published Monday. Earlier clinical trials had established that the shots also prevent illness, hospitalizations, and deaths.
The study adds to evidence that new vaccines made with messenger RNA technology actually reduce the spread of the virus in real-world conditions. An earlier study in Israel found a single dose of the Pfizer vaccine reduced infections by as much as 85%.
The study is "tremendously encouraging" CDC Director Rochelle Walensky said in a briefing, and new findings "underscore the importance of getting both of the recommended doses of the vaccine in order to get the greatest level of protection against Covid-19, especially as our concerns about variants escalate."
While the research bodes well for halting the spread of the virus and speeding economic recovery, health officials remain concerned. Cases, hospitalizations, and deaths are rising again in the US, and the CDC's head cautioned that a new surge is building with much of the country still susceptible.
Walensky said she had a feeling of "impending doom" as the US appeared to be on a similar trajectory to Europe, where Covid spikes prompted fresh restrictions this month.
The US has administered 143 million doses of Covid vaccines, according to the Bloomberg Vaccine Tracker, and more than 22% of the population has been inoculated.
The CDC studied a group of about 4,000 front-line workers, including health-care personnel, first responders, teachers and service workers from mid-December to mid-March as vaccines rolled out widely. These groups were among the first to be vaccinated, along with the vulnerable elderly, because of their risk of exposure to the virus.
Participants were tested for Covid weekly and surveyed for reports of symptoms. The researchers compared the frequency of Covid infections before and after vaccinations to estimate how effective the shots were at preventing SARS-CoV-2 spread, regardless of whether people felt sick or not.
Both the vaccines from Pfizer and partner BioNTech SE and Moderna require two doses spaced weeks apart. Two weeks after the first dose, the shots appeared to prevent 80% of infections; that rose to 90% two weeks after the second dose, when people were considered fully immunized.
The study included a mix of participants who were tested after developing symptoms as well as infections that were picked up by weekly tests when people were still feeling well. The results didn't break out in detail the vaccine's ability to prevent asymptomatic or minimally symptomatic infections.
"The study demonstrates that these two mRNA vaccines can reduce the risk of all SARS-CoV-2 infections, not just symptomatic infections," the CDC said in a statement.
Estimates of how well the shots prevent infection should be interpreted cautiously, due to a relatively small number of infections confirmed, the CDC said. The agency also didn't have enough data in this study to distinguish between the two vaccines. The study took place before a third vaccine from Johnson & Johnson became widely available.
Disclaimer: This article first appeared on Bloomberg, and is published by special syndication arrangement