Infection Rates and Vaccination

Unvaccinated people are about 29 times more likely to be hospitalized with Covid-19 than those who are fully vaccinated, according to a study released Tuesday by the Centers for Disease Control and Prevention.

The new study, published in the CDC’s Morbidity and Mortality Weekly Report, also found that unvaccinated people were nearly five times more likely to be infected with Covid than people who got the shots. (CNBC)

The question is not is it true but how could we know. Hospitalization and death are observable, countable events, but infection is only observed when someone is tested. The U.K. has a program of randomly testing people in order to learn, among other things, how many of them were infected with Covid (REACT-1). But the CDC appears to be just counting people known to be infected.

Vaccination is much stronger protection against a serious case of Covid than a mild case, as shown by the difference between the first paragraph quoted above and the second. One would therefor expect a much larger fraction of vaccinated infections to be asymptomatic. If an infection is sufficiently asymptomatic the victim does not notice and so has no reason to get tested. Unless the CDC is somehow compensating for that problem, its count of the infection rate for vaccinated people is not only too low, it is too low by more than its count of the rate for unvaccinated, hence it is overestimating the protection that vaccination provides against infection.

The effect of vaccination on hospitalization and death rates are what you need to persuade someone that it is in his private interest to be vaccinated — and, on those grounds, I am. The effect on the infection rate is what you need to show that vaccinating one person protects others in order to defend forcing people to get vaccinated. The CDC supports vaccination requirements, which is why I doubt that they have gone out of their way to correct the overestimate of the strength of vaccination against infection that comes out of simply counting known infections. 

What the real protection against infection is I do not know. The estimate from the UK study (ignore the headline) is 50 to 60%, much lower than the CDC claim, but that is a different population and a different mix of vaccines.

A second argument is the claim that someone vaccinated and infected is less contagious than someone unvaccinated and infected. I have seen various claims as to whether that is or is not true, based on measured viral levels in the infected, but we now have something better, a study based on observed infections of people who were close contacts of infected individuals:

Unfortunately, the vaccine’s beneficial effect on Delta transmission waned to almost negligible levels over time. In people infected 2 weeks after receiving the vaccine developed by the University of Oxford and AstraZeneca, both in the UK, the chance that an unvaccinated close contact would test positive was 57%, but 3 months later, that chance rose to 67%. The latter figure is on par with the likelihood that an unvaccinated person will spread the virus. (Nature)

All of which explains why the effect of vaccination on Covid rates has been much less than many of us expected. Death rates have fallen sharply but infection rates have gone up and down, even in well vaccinated populations such as the U.K. and Israel, much as they did before. The most one can say, at least from a casual look at the data, is that the reduced infection rate from vaccination has roughly balanced the increased infection rate from the Delta variant.