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Pfizer half-life 3 months (65-85yo); 3rd booster boost
From an image in Pfizer's late-July investor's report:
with notation added by Michael Lin:
[Image: E7esN_zUcAgXTV0?format=jpg&name=medium]

For 18-55yo, the decrease in neutralizing titers for wild & beta variants matches a half life of 3 months (so reduce by a factor of 16 in a year).

I calculate a half-life of 3.1mo for 18-55 and 2.2mo for 65-85.
That's a yearly drop in neutralizing titer by a factor of 15 for 18-55 and a factor of 44 for 65-85yo.
Got my third Pfizer vaccine 4 days ago. Felt mildly crappy for two days.
My brothers (both MDs and 70+) got their Pfizer boosters in August (after Dec/Jan first shots).  Neither complained about it.

I am waiting on a Moderna booster.  I do know someone who got a Pfizer booster after having had the first two vaccinations being Moderna.  I know of another person whose doctor told them to get a booster (Pfizer) even though they had Moderna before. (I believe US MDs can now prescribe Pfizer for anyone (a 3yo, or a person who received Moderna), so I expect I could get a Pfizer booster if I felt strongly enough.)

I am thankful that the community level of COVID has been coming down (not just hospitalization).  It is still high (85K positive tests a day).  That means I'm less likely to be exposed to COVID, even if my resistance level is dropping.  I did have a scare about 3 weeks ago, when a 5 year old (at home with a respiratory infection of some sort, the 2nd one since school started) coughed straight at me from about 3 feet, outdoors.  I was masked (KN95, and wear glasses); he wasn't.  He had been tested earlier in the day for COVID.  I was relieved when it came back negative.

I do recognize that I'm less anxious about being out and about.  I am spending most days at home (working), but I flew twice in the last month.  I go shopping when I need to.   I went to a gathering for a deceased friend.   But I'm not yet ready to eat at a restaurant (but I get takeout).

In my area (Santa Clara County), most people are still masking.  There were more unmasked people in stores in San Diego county.  There were a few poorly-worn masks at the two airports I was in.

It's depressing when I think that for every 3 people (all ages) in the US, two have been vaccinated and one not.

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