You Can Catch Covid Twice – But it’s Very Rare and Very Mild

Can you catch Covid twice? The challenge trials at Oxford University have now turned their attention to this question, deliberately exposing people who have had the disease before to the virus again to see how their immune systems respond.

Other studies have already looked into this question, though without the controversial deliberate exposure aspect. The most recent, published in the Lancet last week, tested around 3,000 U.S. Marine recruits aged 18-20 for Covid antibodies and then followed them over six weeks while they completed basic training together to see how many became infected. The living in close quarters would likely have ensured that all were exposed to the virus.

The study found that around 10% of seropositive (with-antibodies) participants (19 out of 189) tested PCR positive for the virus versus around 50% of seronegative participants (1,079 out of 2,247). This means that having antibodies from a previous infection gives about 80% protection from testing positive for the virus again. This finding closely matches that of a large Danish study published last month, that found those who had tested positive for the virus in the spring were about 80% less likely to test positive again in the autumn. And also a UK study of NHS workers from January that found being PCR positive for the virus at one point made workers around 80% less likely to test positive again at a later date.

The new study was being used last week to promote the idea of vaccinating young people who had previously been infected, on the grounds that protection via infection was not enough. Thus Sky News reported: “Young people who have already tested positive for coronavirus are not fully protected against reinfection.”

The study itself supported this use, stating its results suggest “COVID-19 vaccination might be necessary for control of the pandemic in previously infected young adults”. Professor Stuart Sealfon of Icahn School of Medicine at Mount Sinai, New York, and senior author of the study, said:

As vaccine rollouts continue to gain momentum it is important to remember that, despite a prior COVID-19 infection, young people can catch the virus again and may still transmit it to others. Immunity is not guaranteed by past infection, and vaccinations that provide additional protection are still needed for those who have had COVID-19.

What such claims appear not to allow for is that questions are being asked about how the balance of risks stacks up for young people to be vaccinated even when they have not had Covid, let alone when they have and have 80% protection already. To this balance must be added that severe side-effects are considerably more common in those who have previously had Covid.

The 80% protection figure is also not the full story on immunity following infection. Noteworthy is that symptomatic infection was much less common among those who had antibodies. In fact, only three out of 19 (16%) seropositive PCR positives were symptomatic, versus 347 out of 1,079 (32%) seronegative PCR positives. The large proportion of PCR positive infections that are asymptomatic even among those without antibodies (68%) may be an indication of the high degree of pre-existing immunity among the young.

The infections among those with antibodies were also much less likely to be infectious, with average Ct of 27-28 versus around 24 for the seronegative infections (Ct or cycle threshold corresponds inversely to viral load, which corresponds to infectiousness). This translates to a viral load about ten times lower, which is considerably less infectious.

If we focus just on the symptomatic participants (i.e., people who are actually unwell), then we find three out of 189 (1.6%) symptomatic infections among those with antibodies versus 347 out of 2,247 (15.4%) symptomatic infections among those without. This corresponds to antibodies giving 90% protection against symptomatic Covid (though three is a very small number to base very much on).

The symptoms experienced by these three and the other PCR positive participants are listed in the study, though no indication is given of severity or duration. Indeed, a major weakness of the study is that 21% of those still in the study once on the base ceased to show up for testing, an unknown number of them because they were transferred off the base for unknown medical reasons, which could have been Covid related. This appears to explain why no data is given on Covid hospital admissions or deaths – anyone in that position would just silently have disappeared from the study!

Overall the findings are in line with what we would expect from immunity following viral infection: 80% reduction in testing PCR positive for the virus (some of which will be false or cold positives), 90% reduction in symptomatic disease, much less infectious (ten times lower viral load). Immunity isn’t binary, and can sometimes involve a mild second bout while fighting off the virus with a primed immune system.

Nothing in this study suggests that the immunity acquired by young people through infection needs topping up with vaccination. The lack of data on severity or hospitalisation means it tells us nothing about the risk of serious illness for those previously infected – though the 90% reduction in symptomatic illness intimates it will be considerable. Vaccination, meanwhile, is not risk-free, with any risks (however small) heightened for the previously infected. Young people, like everyone else, should be allowed to weigh up these risks for themselves and decide whether vaccination is right for them – fully informed and free of any coercive pressure.

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