Two recent studies, one from the UK and the other from Denmark, have addressed the question as to how likely is it that having been infected with Covid-19 once, you can catch it again. The hope is that a previous infection would give sufficient immunity to protect against a second infection. How far is this the case?
What did we know before these studies?
- If infected with Covid-19, most people will develop some immunity
- The level of the immunity has been measured by the level of antibodies produced
- Immunity might be less strong:
- In those who had a very mild infection
- (Rarely) in those who had a very severe infection (which paradoxically dampens the effectiveness of the immune system)
- But we know the level of antibodies:
- Is not the same in everyone who has been infected and is lower, for example, in older people
- Does tend to fall over time following the infection and may even disappear within 6 months
- But we also know that when faced with another infection
- Previous low levels of antibodies (even if they’ve fallen to zero) can rise again in response to a new infection
- Antibodies are not the only defence we have: there are our T cells as well
- As explained before in this blog*, when faced with a second infection, our T cells can also be woken up and attack and destroy the virus
What we didn’t know?
- Despite the data about antibodies, we didn’t know for certain if they would be enough
- Indeed, even if our level of antibody response after a first infection was not that high, the only important statistic is how much less likely is it that they would prevent us getting a second infection
What about the new variants?
- We do know, for example, that the Brazilian variant is not such a great respecter of antibodies from a previous infection as we would like.
- The new UK (Kent) variant though, which is the main variant in much of the world currently, should be covered by previous antibodies; although the UK variant passes more easily from person to person than the original strains
- Thus, there is no reason to believe that there would be any difference in the protection resulting from a previous infection – whether this was the original or the new UK strain
Why answering the question about re-infection is not that easy?
- The obvious question is (as my title suggests): “I have had Covid-19 once, how likely will I get it again?”
- It is obvious that the answer is that reinfection:
- is not “impossible”
- and must be related to how many people around you have the infection
- and how careful you are
- and if you’ve had the vaccine
- and how successful the vaccine is
- The question we now have some answer to is:
- The answer has therefore required some careful monitoring of large populations to make those comparisons; in the UK we have ad the Siren study
- The public health authorities recruited almost 50,000 workers in the National Health Service
- Each of these workers provided swabs and blood tests and were followed up
- The research compared the risk of getting an infection in the second wave between those who had been infected before and those who had not
- This is what they found:
- Let me help in interpreting this graph!
- Look at the dashed line, this is what happened to those workers who had not had any infection first time round
- By the end of the next 6 months, around 5% of them had evidence of a new infection
- Now look at the continuous line, which is what happened to the workers who had a previous infection
- By the end of the next 6 months only around 1% of them had evidence of a new infection
- This is a reduction of 80%
- Of course, we don’t know if the two groups had the same exposure to new infections they were all continuing their roles in the NHS
- Perhaps those with a previous infection were more careful, hence their lower rate
- Equally, they may have been less careful, thinking that they were less at risk because of their immunity
- The conclusion was that previous infection did not take away the risk of a second infection completely but did reduce it (as expected) by a large amount up to 6 months later
The Danish Study
- This was a very similar study
- The headline result was identical: an 80% reduction in the risk of a second infection
- The study also showed that the relative size of this reduction did not go down with time:
- The result thus suggests our immunity can burst back into action
- What was a bit worrying, but not entirely unexpected, was the protection from a previous infection was less in those aged over 65
- The elderly with a previous infection had a much lower rate of protection against a second infection compared to those younger age groups
What does 80% protection mean?
- It is probably worth saying that the absolute risk of having a second infection will still depend on how common the infection is in the general population
- Look at this diagram below, which shows how likely it is someone with a previous infection will get an infection in a second (or say third) wave*
- The different sizes of the pies illustrate the overall number of cases occurring depending on how widespread the infection is
- The orange slice shows that the number of people with a second infection will vary depending on how widespread the virus is
- Although the absolute numbers in the orange slice vary accordingly, their proportion remains the same
What’s the impact of vaccination on interpreting these results?
- Firstly, across all ages a previous infection does not give complete protection against a second infection
- Next, that is more true for people aged over 65, reinforcing the need to have the jab even if you have had a previous infection
- We know that vaccines work, giving at least 80% protection in those over 65, much higher in those who are younger.
- I conclude that vaccines do then give a higher level of protection than natural infection
- What we don’t know is whether a previous infection plus being vaccinated gives more protection than just being vaccinated – I suspect not, but let’s see as the results come in over the next few months
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