The Centers for Disease Control and Prevention (CDC) in America has published a new study claiming to show that, among the previously infected, the unvaccinated are at more than double the risk of re-infection than the vaccinated.
It forms part of their evidence for why people who have had COVID-19 before should get vaccinated and not rely on natural immunity.
It has a number of problems, however.
The study looks retrospectively at data from the U.S. state of Kentucky. The researchers identify all those in the state who were re-infected during May and June 2021 (defined in terms of positive tests a certain number of days apart) and compare their vaccination rates to a control group. They find 246 re-infections in that period, and calculate that those who were not vaccinated were 2.34 times more likely to be re-infected than those who were vaccinated.
The study has a number of limitations, however, some of which the authors acknowledge.
Firstly, the study period of May and June 2021 is notable for being a period of very low prevalence in the state, meaning it is not a good time to study acquired immunity, which is best studied during a new surge when it is most put to the test. The authors themselves acknowledge that because the study covers just one period in one state, the “findings cannot be used to infer causation”.
The authors also acknowledge that test-seeking behaviour may skew the findings to exaggerate vaccine effectiveness, as vaccinated people are probably less likely to get tested.
Another weakness is that the study doesn’t include symptom data so we don’t know whether the “reinfections” were actual disease or just asymptomatic and mild infections of the kind that are characteristic of the immune system working.
A related problem is that there is no discussion of how big the problem of re-infection is from an absolute standpoint. With only 246 re-infections (of unknown severity) in a population of 4.5 million over a two month period, it’s not clear that even if vaccination did halve your probability of re-infection it would be a difference worth getting vaccinated (which carries its own risks) to achieve.
Another problem with the study is that it may be confounded by a number of factors, not just test-seeking behaviour, but also age, for example.
The main claim of the study (about vaccination-plus-infection giving better protection than infection-by-itself) comes from the fact that the unvaccinated made up 73% of those who were re-infected but only 58% of those weren’t re-infected. But this outcome could easily be skewed by the fact that the young are both less likely to be vaccinated and more likely to be exposed, so that the unvaccinated are ‘over-represented’ in the re-infected. Unfortunately, we can’t get any clue as to whether this is what is going on as the results aren’t broken down by age, so we can’t tell whether the re-infections disproportionately occur in the more socially active and less vaccinated young.
If this is the basis on which the CDC is planning to encourage previously infected young people to get the jab to supposedly enhance their immunity then they should be ashamed of themselves. They should at least state the absolute risk reduction (over a range of prevalence levels) so people have a better understanding of the true level of protection the vaccines are giving them (according to the study), rather than just talking in terms of halving a risk that they fail to mention was very small to begin with.
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