When to Get the Booster?

October 3rd, 2023
Let's say you're planning on getting a covid booster this fall: when's the best time to get it? The updated boosters (targeting the XBB lineage) have been out since mid September, so there's not a new vaccine to wait for. Instead, I see the choice as balancing two considerations:

  1. If you get it too soon it might have worn off too much by the time you most need it.

  2. But if you're too late you might get infected in the meantime.

As a first approximation, you probably want to have the strongest protection when local levels be at their highest. When would that be? Wastewater monitoring is pretty good for this sort of thing because it's not dependent on people getting tested. Here's what I see on Biobot:

It looks like 2020-2021 and 2021-2022 were strongly concentrated around New Years, and 2022-2023 less so. On the other hand, 2023-2024 so far is following a trend very close to 2021-2022, so perhaps it will be up for the holidays again?

The other key question here is how quickly the vaccine wears off. It looks like the most recent meta-analysis here is Menegale et. al 2023, which found effectiveness decreased quite rapidly against Omicron (and everything now is a kind of Omicron):

They estimated a half life of 111d [88-115d]. This means that if you got a shot on the first day they were made available this year (2023-09-12) you'd be down to 50% [42-51%] effectiveness at New Years. I wish the CDC would be more transparent about their reasoning so we could tell whether this was on purpose...

At this point I'd love to see a calculator that lets you put in when you last got a booster (or had covid) and then combined the half life data with the historical seasonality data to identify the covid-minimizing time to get a shot. It could even allow you to specify dates you want to not be sick for, or not get sick during, along with how important it is to you.

Unfortunately this calculator doesn't exist, so we'll have to eyeball it. I think most people would like to avoid infection around Thanksgiving and Christmas, historically high-infectious times that we especially don't want interrupted by covid and during which we're much more likely than usual to be getting together in large multigenerational groups. Getting a shot two weeks before Thanksgiving, 2023-11-09, would have you at most protected for Thanksgiving, and then still 82% [78-82%] of peak protection at Christmas. If more worried about infecting other people than getting infected yourself, such as if you're younger but visiting older people, subtract a week to model that you're trying to prevent infection in the week leading up and not during the holiday.

There are a lot of person-specific factors that could affect your decisions. For example, you might be about to travel to see an elderly relative or have an infant, in which case sooner is likely better. Or maybe you had covid recently or have something super important to you later in the season, in which case later could be better. In my case we're doing Thanksgiving early with my wife's family, leaving Boston 2023-11-09, so I'm thinking two weeks before that, less a week for being mostly worried about infecting other people, so around 2023-10-19.

Anything I'm missing?

(I do think it's worth most people getting the booster, even considered selfishly: I'd much rather suffer side effects at a time of my choosing than cancel holiday plans.)

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