A man who boosted his Pfizer COVID-19 vaccine with a shot from Moderna said his side effects were a 'little more severe' the third time round

From left: Study volunteer and virologist Joseph Hyser, Chanei Henry, senior research coordinator of molecular virology and microbiology, and Dr. Robert Atmar, the principal investigator on the study.
Baylor College of Medicine
Scientists in the US are starting a booster shot trial, giving fully vaccinated people another jab.
Participants are given a booster shot of the Moderna vaccine - whether they previously got 2 shots of an mRNA vaccine (from Pfizer or Moderna) or 1 adenovirus (J&J) sh