Covid reinfections: are they milder and do they strengthen immunity?

We’ve known since early on in the pandemic that COVID reinfections could occur. One of the first reinfections reported was in a 33-year-old man from Hong Kong. His initial infection was diagnosed on March 26 2020, with his second infection, with a genetically distinct virus, being diagnosed 142 days later. Since then reports of reinfection have become common, especially since the emergence of the omicron variant. Early research from South Africa (still in preprint, so awaiting review by other scientists) suggests that the risk of reinfection increased quickly and substantially after the variant arrived. So why are reinfections increasing? The simple answer is because our immunity is often no longer sufficient to prevent an infection. This can be due to the appearance of a new viral variant like omicron that, because of mutations to its form, is less recognisable to the immune system, meaning the virus bypasses prior immunity. Or it can be because immunity has waned since we were last infected or vaccinated. We know that this is a particular issue with COVID immunity – hence the need for vaccine boosters. Plus, as I have discussed previously, the coronavirus almost always enters the human body via the nose and throat. Immunity in the mucosal linings of these areas tends to be relatively short lived compared to systemic immunity throughout the body. This may explain why protection against severe illness, usually rooted in the lungs, lasts longer than protection against infection. How common are reinfections? The UK has recently started publishing data on reinfections on its COVID dashboard. It classifies a reinfection as someone receiving a new positive COVID test result more than 90 days after their last infection. Up to February 6 2022, there had been over 14.5 million primary infections and around 620,000 reinfections in England – so one reinfection for every 24 primary infections. Over 50% of all reinfections have been reported in since December 1 2021, suggesting again that the risk of reinfection has increased substantially with omicron. The UK’s Office for National Statistics (ONS) also measures reinfections, although does so differently. For someone to count as being reinfected, there needs to be 120 days or four consecutive negative PCR tests between the positive tests confirming their two cases. The ONS suggests that the rate of reinfection has increased 15-fold since the arrival of omicron, and that currently reinfections account for about 10% of all infections reported in England, compared to just 1% during November 2021. However, I suspect this figure is a significant underestimate. The 90- or 120-day gap will undoubtedly miss some reinfections that occur sooner. Plus, by comparing daily recorded case numbers with estimates of how much of the population is infected with the virus at any given point in time, it appears around half of primary infections are never diagnosed.

Many reinfections are therefore likely to be incorrectly categorised as primary ones. Also, if reinfections are generally milder, a higher proportion are likely to be undiagnosed.

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