A rare side effect that can occur and affects around 1 in 1,000 people may be temporary one-sided facial drooping.
Some provider offers are subject to change and may have restrictions.
While related to vaccination, this adverse drug reaction is generally mild and self-limited and is unlikely to impede a booster vaccination program.
However, the Pfizer booster was found to cause milder symptoms than the Moderna booster in the Spanish report.
Reports of a headache also rose from 42% to 52% from the first to second dose, while reports of muscle pain rose from 21% to 37%.
The extra dose would be given once they are at least six months past their last Pfizer shot.
In total, 44 out of 306 participants had at least one unexpected side effect, like swollen lymph nodes, reports.
Scientists don't expect boosters to yield worse side effects than the second shot, nor any higher risk of severe allergic reactions.