First dose of Pfizer COVID-19 vaccine protects against asymptomatic infection, UK study finds

A new study out of the UK suggests the first dose of the Pfizer-BioNTech COVID-19 vaccine could significantly protect against asymptomatic infection. 

Researchers tested daily thousands of healthcare workers at Cambridge University Hospitals NHS Foundation Trust who were reporting well to work, some who had been vaccinated and some who had not. 

Researchers split the vaccinated workers into two categories - those who were less than 12 days post-vaccination and those greater than 12 days. 

During the study, 26 unvaccinated healthcare workers tested positive for COVID-19, which is 0.8%, compared to 13 vaccinated healthcare workers, or 0.37%. These vaccinated healthcare workers had gotten the shot less than 12 days ago.

Once at least 12 days had passed since getting the shot, only four vaccinated healthcare workers tested positive, or 0.2%. 

The study paid close attention to those in this group for infection, as the 12-day threshold is when study authors say protection against symptomatic infection began to appear in phase three of the vaccine's clinical trial.

Researchers said this study's data suggests the risk of asymptomatic COVID-19 infection decreased by four times among participants who got their vaccine at least 12 days ago. Researchers said they found the vaccine had an "intermediate effect" on asymptomatic infections within the first 12 days. 

In general, if vaccination doesn’t completely prevent infection, it will significantly reduce the amount of virus coming out of your nose and mouth – a process called shedding – and shorten the time that you shed the virus. This is especially important in asymptomatic patients who wouldn’t know they’re "shedding" the virus.

With two doses, the Pfizer vaccine has been proven to be 95% effective against symptomatic COVID-19 infection. 

The U.K., where the study took place, has implemented a contentious strategy of delaying the second shot so it can protect more people quickly with a first dose — a decision that has been criticized as risky by other European countries.

Pfizer recommends that its shots be given 21 days apart.

RELATED: Fauci says US will 'go by the science,' not delay 2nd vaccine dose as UK takes different approach

Earlier this month, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the U.S. will "go by the science" from its own clinical trials and continue administering second doses of the Pfizer and Moderna vaccines within the recommended timeframe. Moderna recommends 28 days between doses. 

Both the Pfizer and Moderna vaccines have been approved in the U.S. by the FDA for emergency use. J&J’s single-dose vaccine was greenlighted on Saturday

Pfizer has not endorsed the decision to space out the time between doses. 

This story was reported from Detroit. The Associated Press contributed.