“Dr. Grove, I got COVID,”  was the first thing she said to me when I returned her call.  She has severe asthma and she was a bit worried.  Fortunately, she did not have severe a case so the first thing she said did not worry me.  It was the second thing she said that worried me.  “I got my vaccine, too”.  My next thought was that she must have contracted her infection between the first and second vaccine doses and, as if she read my mind and before I could ask, she concluded, “And I got my second dose in February.”  Ugh.  This means she got COVID even though she was fully vaccinated.  That wasn’t the only call like this I got last week.

The second call was from someone who wasn’t feeling well.  She called me to ask what she should do since she was exposed to someone who was known to have COVID.  The person she was exposed to was fully vaccinated but had more than one child at home with proven COVID.  She felt sick but assumed that it couldn’t be COVID because she was fully vaccinated.  She didn’t get tested or quarantine and went to the party even though she didn’t feel well.  Unfortunately she was wrong and tested positive a few days later after exposing multiple people. 

The mistake that she made was the result of a flaw in human cognition that makes us prone to a particular type of error. Humans tend to make more mistakes in our estimations of probability when those estimates are at the extremes.  As an example, if there is a 5% chance of something we behave as if the chance is 0.  If there is a 95% chance of something we behave as if it’s a guarantee.  Wouldn’t you be upset if the rain came down on your parade when the weatherman said there was only a 10% chance of rain?  The weatherman did not get it wrong. 

This is the mistake that people are making when evaluating the risk of getting COVID after being vaccinated.  The vaccines are a victim of their own success.  We all heard the reports of 95% efficacy and we all cheered but many have made the following two mistakes.

First is that many are confusing the difference between efficacy and effectiveness.  Efficacy is how well the vaccines work in a clinical trial while effectiveness is how well they work in real life.  In a clinical trial there is a team of assistants and researchers assuring that every vaccine is given perfectly (which is especially an issue with the mRNA vaccines because they must be kept very cold).  In real life it doesn’t always work this way which makes effectiveness never as good as the efficacy.  We saw this in the observational data from Israel that I discussed previously.  The effectiveness under real world conditions was under 90% which is still amazing just not as good as the trials. There is also is concern that the new variants will lower the effectiveness even more, although the vaccines seem to be holding up against the B1.1.7 UK variant. Many are thinking the vaccines are 95% effective but they are not (although they are pretty close).  The second problem is that people heard the number 95% and are treating it as if it is 100%. 

To really understand the risks after vaccination you need to look at the issue from two perspectives. 

First is from the perspective of the individual.  If you are vaccinated and we assume an effectiveness of 85-90% this means that there is a 10-15% chance that you are not protected.

The second perspective is that of the crowd.  If you have a group of 100 people, 10-15 % of them are not protected.

This is driving the current recommendations from the CDC and others.  They have indicated that it is relatively safe for vaccinated people to gather in small groups with low risk unvaccinated individuals from a single household.  This is because, in small numbers, the chances that someone will have COVID and spread it widely or that someone will become severely ill are very low. 

Do not fear, this does not mean that we are going to wear masks forever.  

When the numbers are larger, the risk increases.  Imagine you have a room with 200 people, 150 of which are vaccinated and assume 85% effectiveness.  Of those 150 about 130 are protected leaving 70 vulnerable individuals.  One infected individual could create a super spreader event under these conditions which is enough to warrant the recommendation to keep masks on.

The problem is made worse by the fact that vaccinated people are assuming they can’t get COVID again, which is a mistake.  Even if the vaccine is 90% effective, 90 isn’t 100.  This is creating a scenario where vaccinated individuals who get COVID do not get tested and do not quarantine allowing them to spread the virus more efficiently.

Do not fear, this does not mean that we are going to wear masks forever.  This is where herd immunity saves the day.   Imagine the vaccine rates go up higher.  Now imagine that room of 200 people with 190 are vaccinated leaving only 29 vulnerable individuals.  If there are no (or very few) cases in the community, the chance that one of those 29 people is infected is low enough that we can be without masks altogether.  The lower the case rates the more unvaccinated people there can be without spreading the virus.

So how should we use this information:

  1. If you were vaccinated and develop COVID symptoms, do not assume that, since you are vaccinated,you couldn’t possibly have COVID.  Yould should still get tested and quarantine until you get a negative PCR test.
  2. Keep wearing masks in crowded places.
  3. Don’t assume the pandemic is over until you see the case numbers in your community fall to low levels and stay there for a couple weeks.

The decline in case numbers is not out of our control.  The more people get vaccinated and the more people stick to the recommended mitigation strategies the faster we’ll get there.  Just hang on a bit longer. 

3 thoughts on “90 is not 100 and the risk of COVID after vaccination

  1. Is there or will there be any data on fully vaccinated people who get COVID and end up having “Long Haul” COVID?


  2. Why is the data on “breakthrough” cases so poor? It is very difficult to take calculated risks when there are no reported numbers from which to make those calculations. We are planning to reopen a social dance to vaccinated persons only, which does seem pretty safe. For two days in our region, we got to the 1:100,000 level of new cases, Now the new case numbers are back above 4:100,000 in a region where more than 60% have been vaccinated. Assuming that there is no official reporting of breakthrough vs. non-vaccinated new cases, at what level of new cases must we recognize that the vaccines may not be that effective against the newer variants? My seat of the pants sense is that an escalation over the next few weeks to a community spread number of 10:100,000 would cause me concern, but I have no data to back that up. Thanks for sharing your thinking.


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