For those who initially received the Moderna or Pfizer vaccine, officials said one generally should stick with the same vaccine series, primarily because there is no evidence of marked benefits, though the FDA announcement made no recommendation to that effect.
Some reasons to switch to a different brand might be if the individual had a particularly bad reaction to the mRNA technology included in both Moderna or Pfizer vaccines or if availability of their initial series is limited. A preliminary study discussed by an FDA advisory committee showed switching would be all right if the initial brand isn’t currently available.
However, for those who took the one-dose J&J vaccine, early research suggests taking the Moderna vaccine as a booster yielded the greatest antibody increase. The study showed that a second shot using the Moderna vaccine triggered a 76-fold increase in antibody levels. By contrast, a Pfizer booster increased antibody levels 35-fold, and the coordinating J&J booster yielded only a fourfold increase.
“If the FDA recommends mix and match, then I think the most promising thing is that people who’ve got the J&J would have the ability to switch over to Moderna or Pfizer for their boosters,” Tanna said.
Safety concerns may also play into some J&J recipients’ decision to switch to an mRNA booster shot. According to Yale Medicine, out of 9 million doses administered by May, 28 people developed a rare blood clotting disorder, three of whom died. The side effect tended to be slightly more common in women under 50 years old.
In July, the FDA also attached a warning for a rare neurological condition called Guillain-Barré syndrome to the J&J vaccine. Again, the cases were exceedingly rare with about 100 suspected cases primarily among men aged 50 years and older.