Full Version: Risk of contracting/spreading COVID if fully vaccinated
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I have mentioned  Steven Shafer's COVID reports previously.  This Monday's posting contains an analysis of the (high) likelihood of getting COVID if fully vaccinated.  This analysis expresses some things I've been trying to point out.

TL;DR:  "if the exposure rate in the Pfizer trial was one exposure in every 10 subjects, then a fully vaccinated person has a 50% chance of getting COVID-19 after just 100 exposures."

As he says we are free to share this freely,  I will repost that part.

  1. I was contacted by a friend today, a Professor at a University in NYC, who had attended an international meeting several months after receiving two doses of the Pfizer vaccine. He is now in isolation abroad having contracted the Delta variant (PCR confirmed) somewhere during his travels. The symptoms (running nose, headache, sore throat, a wet cough) have persisted for 9 days. Particularly shocking to my friend was that he had no idea he had COVID-19 until he was swabbed prior to boarding a plane to return to the US. Being vaccinated, he thought he was immune, and this was likely just a common cold. Meeting with his unvaccinated colleagues over several days, all of whom were unmasked thinking they were safe, he repeatedly exposed them to COVID-19, just as he likely got it from a vaccinated colleague who thought he or she just had a cold. However, it is no cold – it’s the Delta variant in a fully vaccinated individual.
In multiple emails today (he’s holed up in a hotel) we discussed the poorly appreciated risk of COVID-19 in vaccinated individuals. Being vaccinated, and with guidance from the CDC, I have been much more relaxed about going to restaurants, being in crowds, not wearing a mask when meeting other physicians in lounges at Stanford, etc. That is a big mistake. If I keep this up, I’m going to get COVID-19, and likely spread it.
This reflects a new appreciation for what the vaccine does and does not do. The vaccine is 95% effective against getting COVID. I’ve attached the Pfizer FDA Briefing document. The 95% efficacy appears in Table 13 on page 32. This is correct. This is the reason to get vaccinated. We know from subsequent studies that it is almost 100% effective against death.
95% efficacy tells you nothing about the risks of getting COVID if you are vaccinated. 95% is the efficacy relative to unvaccinated individuals. The actual risk of getting COVID in the Pfizer study is a function of the exposure rate. We don’t know what that rate is. The trial involved about 12,000 subjects per group (I’ll spare you the math). How often was each subject exposed to COVID-19? We don’t know.
Let’s say that each subject was exposed exactly once. According to table 13 on page 32 there were 172 cases in the unvaccinated cohort and 9 cases in the vaccinated cohort. Given 1 exposure per subject in the study, we can calculate the probability of NOT getting COVID-19 as (12000-172)/12000 per exposure in the vaccinated group, and (12000-9)/12000 per exposure in the vaccinated group. After N exposures, the chance of NOT getting COVID-19 can be calculated as ((12000-172)/12000)N in the unvaccinated group, and (12000-9)/12000)N in the vaccinated group. Here is a figure showing the number of exposures and the chances of getting COVID-19. (Here is the spreadsheet:!AuOyHP_aTIy7ttZ5sMv3...g?e=T4rS4T).
[Image: image034.png?part=0.0.33&view=1]
After 1000 exposures, fully vaccinated individuals have a 50:50 chance of getting COVID-19. One thousand exposures may be a couple of trips through an airport, or a few months of dining out. However, that assumes that each individual in the Pfizer trial was exposed to COVID-19 exactly once. How about if only 1 in 10 were exposed to COVID-19 during the trial? Here is what that looks like:
[Image: image036.png?part=0.0.34&view=1]
As you can see, if the exposure rate in the Pfizer trial was one exposure in every 10 subjects, then a fully vaccinated person has a 50% chance of getting COVID-19 after just 100 exposures. That could be a single plane flight. It is nowhere nearly as much protection as I had assumed from being fully vaccinated.
I don’t know the actual number of exposures in the Pfizer trial. I would welcome any thoughts on how to better understand the overall risk reduction in fully vaccinated individuals. However, what is clear is that:
  1. The actual risk reduction with vaccination is much smaller than we all believe. A fully vaccinated individual will have a 50% risk of getting COVID-19 with as few as 100 exposures if only 1 in 10 subjects in the Pfizer trial was exposed. Damn….
  2. It’s great that fully vaccinated people will probably only get a bad cold. However, that may increase the likelihood that fully vaccinated people will contribute to the spread of COVID because they will assume the illness is something else because they are vaccinated. They will not isolate, wear masks, and may become super spreaders.
  3. The Delta variant is at least twice as infectious as the Alpha variant that was circulating at the time of the Pfizer trial. The viral loads with the Delta variant may be as much as 1000 fold higher viral than in subjects with the Alpha variant. An encounter that might not have been an “exposure” with the Alpha variant may well be an exposure with the Delta variant. Between the surge in the Delta variant and the relaxation in restrictions, exposures are going up, not down. 
  4. The combination of relaxing non-pharmaceutical interventions, having vaccinated people believe that they are safe from infection, an increase in mild infections in fully vaccinated individuals, and a highly infectious variant as the dominant strain likely explains the disaster unfolding in England. The rise in deaths is in unvaccinated individuals, but the rise in cases may be driven by high exposure rates in vaccinated individuals.
It ain’t over ‘till it’s over. We need to re-evaluate the wisdom of relaxing non-pharmaceutical interventions, both as a society and as individuals, in the face of a highly infectious variant. In the UK, 40% of the hospitalizations are in fully or partly vaccinated individuals (see Even CNN is now talking about the worrisome possibility that fully vaccinated individuals are getting COVID-19 (see However, they say this is “rare.” Over the weeks ahead, it may become common, depending on individual behavior.