I’ve had Covid once, will I get it again?

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
*didn’t want to confuse but the blue slices of pie refer to how many cases those previously infected would have had if none of them had any protection

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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2 replies on “I’ve had Covid once, will I get it again?”

Hi Alan
Enjoying your blog, especially the studies you find. The real question for the future will be what is the risk of catching Covid-19 when two people who have had the vaccine and the booster meet?
Best wishes
Rob Pittack


Thanks Rob. The unknown is how far will the vaccines stop you contracting CoVid as opposed to stop you getting sick with the virus. So basically even if you are vaccinated, if you were exposed to the virus, it could multiply enough for you to pass it on to someone else and so on without anyone getting ill. My reading suggests that this is probably reduced by about 2/3 in those who are vaccinated. I am not that worried about this risk of transmission, if as seems likely it will predominantly be asymptomatic. The Pfizer adolescent study press released today seems to support that.


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