Natural Immunity to One Variant Protects Against Severe Disease From Another

We already have evidence that natural immunity provides better protection against infection than the vaccines. And since you can’t get severe disease if you don’t get infected, this implies that natural immunity also provides better protection against severe disease.  

Of course, if you did get severe disease the first time around, it may not be much consolation to know that you now have strong protection against that outcome. However, the fact that natural immunity provides strong protection against severe disease should still be considered good news.

Even if you had severe disease once, it’s reassuring to know you’re less likely to have it again. And since different variants of Covid have different properties, it’s possible that a bad bout of Covid caused one variant might protect against a worse bout caused by another.

But what degree of protection are we talking about? A recent study from Qatar, published in the New England Journal of Medicine, has some answers.

Using a large national database, the researchers identified cases of severe, critical and fatal disease caused by primary infections and reinfections from March of 2020 to April 2021. Individuals who’d been vaccinated were excluded from the analysis.

Primary infection was defined as the first positive test; secondary infection as the first positive test obtained at least 90 days after the primary infection. To compare like with like, the researchers matched individuals with reinfections to those with primary infections by age, sex, nationality, and week of the test date.

Of the 1,304 reinfections, 32% were caused by the Beta variant; 4% were caused by the Alpha variant; and 16% were caused by the original Wuhan strain. The remaining 48% were of unknown status.

However, given that Alpha and Beta were overwhelmingly dominant by the time Qatar experienced its second wave, and the fact that natural immunity typically lasts for more than a year, it’s likely that the vast majority of the “unknown” reinfections were caused by either Alpha or Beta.

Results are shown in the table below. The reason why the totals are much greater in the right-hand column is that the researchers matched individuals in a ratio of 1:5 to get a larger sample size.

The results are presented as odds, but percentage are easier to understand. Looking at the final row, the percentage of reinfections that led to severe, critical or fatal disease was 0.3%, whereas the corresponding percentage of primary infections was 3.1% – about ten times higher.

This suggests that natural immunity to the original Wuhan strain reduces the risk of severe disease from Alpha or Beta –  conditional on infection – by about 90%. And if you assume that Alpha and Beta are more virulent than the original Wuhan strain, the risk reduction is even greater.

Widespread natural immunity is more than capable of defanging Covid. Which is why it should have been part of our strategy from the very beginning, along with focused protection for high-risk groups. In Britain, it actually was part of our strategy… until the Government went for lockdown instead.

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November 2022
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