The first of 600 trial participants have received an investigational Pfizer pneumococcal vaccine following a third, or booster, dose of the drug company’s approved COVID-19 shot.

The participants — all aged 65 years and older — will be followed for six months to assess the safety of co-administering the two drugs and to track each vaccine’s effect on immunity, Pfizer announced Monday. They all will have received their second vaccine dose at least six months before receiving the co-administered pneumonia shot and COVID-19 booster. 

The 20-valent pneumococcal conjugate vaccine (20vPnC) candidate protects against 20 serotypes responsible for most pneumococcal disease currently circulating, according to Pfizer. The 20vPnC drug has been approved for review this summer by the Food and Drug Administration, the company added.

Whether a COVID-19 booster shot will be needed and when is not yet known, according to federal health officials. But it is possible that the immune response to the three COVID-19 vaccines approved for use in the United States will wane over time, they say. 

“As the mean of the number of people who are vaccinated start[s] to come down and reach the point where you’re getting close to that critical level, then you’ll seriously consider the possibility of a booster, but I don’t know when that will be,” Anthony Fauci, M.D., chief medical adviser to the Biden administration, told the Washington Post on May 20. “But it would be foolish for us to not at least consider that we may need that.”

The Food and Drug Administration in February pledged that it would fast-track approval of booster drugs to strengthen COVID-19 vaccine protection against virus variants that have health officials concerned, the Wall Street Journal reported at the time.

U.S. COVID-19 rates currently are at their lowest in 11 months, likely due to the vaccine rollout, according to health officials.