I’m traveling and will need a booster. Already took one JJ shot. What to do?
Wish I could tell you definitively. My friend and I got the J&J last May. He's about 70. In November his liberal friends convinced him to get a booster. Don't remember which but it was Moderna or Pfizer. It literally knocked him flat on the floor for a couple hours. He eventually got back to normal (which means his normal semi-disabled self), but it took a week or two.
If it's possible, find a second J&J with a doc who sees the need for a matching shot. Otherwise get the German guy who took 60 shots for other people to take yours.