Wed 2021-Apr-07

Immunity freedom day!

Tagged: COVID

As of today, your humble Weekend Editor is 2 weeks past the 2nd dose of the Pfizer vaccine. At 5pm, I am officially out of COVID jail! Right? Not quite…

What it means once you’re fully vaccinated

Recall that a vaccine efficacy of 95% means that I now have 5% of the probability/unit time of getting COVID-19 as an unvaccinated person in similar exposure circumstances:

\[\mbox{efficacy} = 100.0 * \left( 1 - \frac{\Pr(\mbox{infection} | \mbox{vax})}{\Pr(\mbox{infection} | \mbox{non-vax})} \right)\]

It means my risk is dramatically reduced (by 95%) compared to no vaccine at all, given similar exposures. It does not mean I’m bullet-proof. In particular, it does not mean that I can engage in COVID-risky behavior with wild abandon!

Also, since the vaccine suppresses serious COVID (hospitalization/death), I could nonetheless be an asymptomatic carrier and thus a danger to those around me who are as yet unvaccinated. Vaccines are about protecting yourself, and about not being a jerk to everyone who comes near you, too.

That will change once herd immunity is attained (assuming it can be attained, in spite of stubborn-stupid vaccine resistance.

The real reason

Of course, I have an ulterior motive. No, nothing about being a scientist doing drug research in the past; take your Big Pharma conspiracy theories elsewhere. Trust me: they were smart, but not that smart.

No, my real ulterior motive: the Weekend Editrix doesn’t reach her Immunity Freedom Day until May 5.

After that, we can start looking at immunity numbers in the US (COVID convalescents + vaccinees), and decide about the safety of a summer holiday. Though, truth be told, I really, really miss the Club Med in the Bahamas, but that’s not open until December 2021. Anybody want to join us there in December or January?

Notes & References

1: Nope, no time for notes today. Out makin’ antibodies & teachin’ stuff to memory B cells.

Published Wed 2021-Apr-07

Gestae Commentaria

David Pablo Cohn, Thu 2021-Apr-08 14:12

Coming up to my second jab next week (yay!), I found myself wondering about the nature of that 95%. I don’t think it’s a question that we can answer, given available data, but I can imagine that number manifesting in two different ways:

1) Everybody gets immune defenses that are 95% effective - e.g. expose them 20 times, and on average the virus will get through once.

2) 95% of the population gets bulletproof immune response, but 5% get nothing.

I’m sure everyone’s immune responses vary, so it’s a combination of the two, but I can’t help but wonder what that curve looks like.

Weekend Editor, Thu 2021-Apr-08 14:31

Yes, that’s right: the vaccine efficacy as measured in the clinical trials is a population measure, not an individual one. We can’t distinguish between average risk (your alternative 1) vs subpopulation risk (your alternative 2).

More about efficacy and confidence intervals here, complete with worked numerical examples, R scripts, plots, and lions & tigers & bears, oh my!

And BTW, the J&J South African trials found vastly different efficacy between the HIV+ and HIV- populations (49% for HIV+, 60% for HIV-), so we know that alternative 2 can in fact occur for people whose immune system has been messed up.

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