Pfizer has administered its first COVID-19 booster shots.
The pharmaceutical company is studying a third dose of the vaccine and a pneumococcal vaccine candidate.
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The first participants received their shots Monday.
The trial is looking at safety when the vaccines are co-administered and will follow up six months after.
Pneumococcal vaccines help prevent illnesses like pneumonia.
Six hundred adults over the age of 65 are in the trial.
Some will receive the COVID booster and pneumococcal vaccine, while others will only receive one and a placebo.