The Pfizer and Moderna COVID vaccines were shown to be 90 percent effective against coronavirus infection — regardless of symptom status — in a real-world study involving almost 4,000 American health workers that was published Monday.
In further encouraging news, partial vaccination with one dose of either vaccine resulted in 80 percent protection against infection two weeks after the first shot.
The study conducted by the Centers for Disease Control and Prevention (CDC) looked at the effectiveness of the two vaccines among 3,950 participants across six states from December 14 2020 to March 12, 2021.
"This study shows that our national vaccination efforts are working," said CDC Director Rochelle Walensky.
"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."
One of the big strengths of the study was that participants self-collected nasal swab tests each week for lab testing, regardless of whether they developed symptoms or not.
This adds to a growing body of evidence that the vaccines halt not just symptomatic disease but infection itself, making them an important tool in stemming the spread of the virus.
The participants included doctors, nurses, first responders, and other health care workers from Arizona, Florida, Minnesota, Oregon, Texas, and Utah.
Among them, 2,479 (62.8 percent) received both recommended mRNA doses and 477 (12.1 percent) received only one dose of mRNA vaccine.
There were three positive cases among the fully immunized, which translated to an incidence rate of 0.04 per 1,000 person-days, contrasted to 0.19 per 1,000 person-days for partially immunized, and 1.38 per 1,000 person-days for the unvaccinated.
After adjusting the model to account for the location, this came out to vaccine effectiveness at 80 percent for one dose and 90 percent for two.
The study's authors said they could not make product-specific estimates because of the limited number of infections.
The study is ongoing, and scientists will look to sequence the virus in cases where it was able to infect people despite vaccination, to better understand why this happens in some cases.
