Pfizer vaccine shows 94% effectiveness against asymptomatic transmission of COVID

Pfizer's vaccine is successful in preventing not only symptomatic COVID-19, but also asymptomatic disease according to new real-world data, Israel's Ministry of Health and Pfizer/BioNTech announced Thursday.

As concerning COVID variants spread and the companies behind the three authorized vaccines hurry to test their shots against them, there's other promising news from Thursday's announcement: this latest analysis was performed when more than 80% of Israel's COVID-19 cases were from the UK variant B.1.1.7 -- demonstrating that the Pfizer vaccine is equally effective against this variant, which is known to be more contagious, and possibly even more deadly, Israel's Ministry of Health reported.

The announcement included a key statistic related to an alarming way the virus can spread -- via people who are asymptomatic, who may not even know they're contagious. The Pfizer vaccine is so far 94% effective at preventing this type of infection, Israel's Ministry of Health reports -- encouraging news that the vaccine could help slow silent transmission.

The Pfizer clinical trials were designed to measure how well their two-dose vaccine worked to stop symptomatic cases of the disease -- cases where people knew they had the virus, including severe cases leading to hospitalization or death. Israel's most recent announcement cites the Pfizer vaccine as 97% effective at preventing these sorts of cases -- which roughly matches the 95% efficacy Pfizer noted in its trials.

Israel's latest data, yet to be peer reviewed, was collected two weeks after administration of the second vaccine dose, reinforcing the idea that both doses are needed to achieve the full efficacy they report. And those who did not receive the vaccine were up to 44 times more likely to develop symptomatic disease, and 29 times more likely to die from COVID-19.

"For the people that have already been vaccinated, it's another reason why it's so important that we've already rolled up our sleeves and received the vaccine," said Dr. Todd Ellerin, director of Infectious Diseases at South Shore Health. "And for those who are vaccine hesitant, hopefully data like that shows that these vaccines are unbelievably effective at not just preventing severe disease, but also markedly reducing infection. And if you can reduce infection, you will reduce transmission."

Eric Silberman, MD, a resident physician in internal medicine at Northwestern Memorial Hospital, is a contributor to the ABC News Medical Unit.