So I will start with a belief I have, formed from the bulk of the research I have read and looking at Minnesota and other data. I think it is fairly clear that being infected provides a longer-lasting, stronger response to a subsequent CV-19 infection attempt than does being vaccinated. There is some evidence that a combination of vax and infection may provide yet greater protection, but that isn’t clear to me from the research. Having a prior infection also doesn’t keep you from getting reinfected and we have seen lots of reinfections over the last year. I will opine again that one of the reasons the immune response to CV-19 isn’t that strong and long-lasting is that the body simply doesn’t perceive it to be that great a threat. It would be nice to have data on the overlap of reinfections and breakthrough infections, but Minnesota DOH, which has the data, won’t release it. I have no idea why.
I have pointed out multiple times how a proper vaccine effectiveness trial needs to be structured, which is getting close to impossible to do given current infection and vax rates in the population. Aside from the usual consideration of various demographic and health status variables, you would divide people by vax and infection events and track the time from each such event. You would verify infection status not just by test results but by antibody screening. If you can find them, you would ideally have a group that has never been infected or vaxed, your pure unvaxed group. You would also have a group that is vaxed but never infected, also very hard to find at this point. And a group that has been infected but never vaxed. Good luck finding those sub-groups. In any event, you would have a very large total population to work with, which you would track, as I said above, by vax and infection event, and compare relative rates of subsequent infection taking time from the last vax or infection event into account. You would also be looking at severity of the infection.
That is all lead-up to describing a pretty interesting vax effectiveness study, in the elderly, from Canada. In addition to looking at vaccination, the researchers took prior infection into account as well. The primary outcome measured was hospitalization during the Omicron waves. In regard to hospitalization, against early Omicron strains, two doses of vax were 78% effective (meaning those persons were 78% less likely to be hospitalized) while four doses had a 96% effectiveness. Against late Omicron strains, two doses were only 40% effective and four doses 68%. This is likely due to both a lessening of immune response and greater evasiveness in the later strains.
For those with a previous non-Omicron infection, protection against an early Omicron strain hospitalization was 88%, while a previous Omicron infection afforded 96% effectiveness. For late stage Omicron the effectiveness was 69% from a previous non-Omicron infection and 90% from a previous Omicron infection. As with the vax, there likely is a lessening of immune response from a prior infection over time. But you will note that in all cases, prior infection appears more protective than vax alone.
For those with a prior infection and vax, protection against hospitalization exceeded 90% for at least 6 to 8 months. For those with a prior Omicron infection alone, no vax, protection against hospitalization exceeded 80% for that 6 to 8 month period.
This study was among those 60 and over, I would anticipate even greater effectiveness in younger groups for both vax and prior infection. Given overal infection rates and the superior protection afforded by infection, one of my takeaways continues to be the limited benefit of any additional vax doses. And as the authors note, they likely missed a large number of prior infections due to home testing or just no testing, which would understate the benefit of a prior infection or hybrid immunity. The researchers also note the low rate of prior infection reported among the unvaxed group. This also may be due to home testing or just ignored infections or a behavioral effect if this unvaxed group more rigorously isolated itself. (Medrxiv Paper)