Edit (November 2022): comments raised some probably valid concerns about my sources. I also don't currently believe it's possible to buy the kind of zinc lozenges studied; they're too unpleasant, what you buy in a drug store has been changed for palatability.
Zinc lozenges are pretty well established to prevent or shorten the duration of colds. People are more likely to get colds while travelling, especially if doing so by plane and/or to a destination full of other people who also travelled by plane. I have a vague sense you shouldn’t take zinc 100% of the time, but given the risks it might make sense to take zinc prophylactically while travelling.
How much does zinc help? A meta-analysis I didn't look at very closely says it shortens colds by 33%, and that’s implied to be for people who waited until they were symptomatic to take it: taken preemptively I’m going to ballpark it at 50% shorter (including some colds never coming into existence at all). This is about 4 days, depending on which study you ask.
[Note: only a few forms of Zinc work for this. You want acetate if possible, gluconate if not, and it needs to be a lozenge, not something you swallow. Zinc works by physically coating your throat to prevent infection, it’s not a nutrient in this case. You need much more than you think to achieve the effect, the brand I use barely fits in my tiny mouth.]
Some risk factors for illness in general are “being around a lot of people”, “poor sleep” and “poor diet”. These factors compound: being around people who have been around a lot of people, or who have poor sleep or diet, is worse than being around a lot of well-rested, well-fed hermits. Travel often involves all of these things, especially by air and especially for large gatherings like conferences and weddings (people driving to camp in the wilderness: you are off the hook).
I struggled to find hard numbers for risk of infection during travel. It’s going to vary a lot by season, and of course covid has confused everything. Hocking and Foster gives a 20% chance of catching a cold after a flight during flu season, which seems high to me, but multiple friends reported a 50% chance of illness after travel, so fill in your own number here. Mine is probably 10%.
If my overall risk of a cold is 10%, and I lower the duration by 50%/4 days, I’ve in expectation saved myself 0.4 days of a cold, plus whatever damage I would have done spreading the cold to others, plus the remaining days are milder. Carrying around the lozenges, remembering to take them, and working eating and drinking around them is kind of inconvenient, so this isn’t a slam dunk for me but is worth best-effort (while writing this I ordered a second bottle of zinc to sit in my travel toiletry bag). It’s probably worth a lot for my friends with a 50% risk of illness, have unusually long colds, or live with small children who get cranky when sick. You know better than me where you fall.
Things that would change this cost-benefit estimate:
- Seasonality
- Personal reaction to zinc, or beliefs about its long term effects
- Covid (all the numbers I used were pre-covid)
- Different estimates for risk of illness during travel
- Different estimates for the benefit of zinc
- Personal susceptibility to illness
Caveats: anything that does anything real can cause damage. The side effects we know about for zinc lozenges are typically low, but pay attention to your own reaction in case you are unlucky. I remain an internet person with no medical credentials or accreditation. I attempt to follow my own advice and I’ve advised my parents to do this as well, but sometimes I’m rushed and forget.
ETA: I originally wrote this aimed at friends who already believed zinc was useful but hadn't considered prophylactic use, and as such didn't work very hard on it. I mistook some rando meta-analysis for a Cochrane review, and didn't look further. There's a pre-registered study that has come out since showing no effect from zinc. There could be other studies showing the opposite, I haven't looked very closely. Plausibly that makes publishing this irresponsible- you definitely should judge me for mistaking a review that mentioned Cochrane for an actual Cochrane review. OTOH, writing too defensively inhibits learning, and I want to think my readers in particular are well calibrated on how much to trust off the cuff writing (but I hindered that by mislabeling the review as from Cochrane).