What is the risk of catching Covid-19 twice?

Headline in the BBC News today was a report from the UK epidemiological study REACT that the level of antibodies against Covid-19 was falling in the UK population, suggesting that the protection from antibodies against future infection might be short lived. This is not necessarily the case but today I examine the recent evidence that Covid-19 can be caught twice.

What did the REACT study find?

  • This study (discussed before in this blog) surveys random samples of the UK population and these new results are based on blood tests taken in July, August and September 
  • It might be expected that the percentage who had antibodies should have gone upassuming that those who had antibodies in July did not ‘lose’ them and should have been added to by those identified in the two later months 
  • In fact as the graph shows, the percent with antibodies has fallen around 25% over the 3 months
  • We know that July and August were relatively low incidence months so major increases may not have been expected.
  • The report implies that those who tested positive for antibodies in July were likely to have lost these antibodies two months later
  • We have to be careful as it was not the same people who had been tested each time and this may account for some of the difference
  • More importantly this is not necessarily new information.  As readers of this blog will know I pointed out 3 months ago* that we knew immunity did wane over time but that did not necessarily mean that when faced with a second infection the body’s immune defences might not leap into action.    * 

What do we know about the numbers who have caught Covid-19 twice?

  • Obvious, but worth repeating, we only know about the risk in the relatively short period of time since the start of the pandemic –  around 9 months
  • Any conclusion, whether reassuring or not, can only apply to this short time frame
  • There are no robust national data of the rates of a second infection, due in large part to what seems to be very low numbers of such cases
  • There have been reported in the scientific literature around 24 cases around the world who have had a second infection (compared to the over 40 million of diagnosed cases)
  • There may have been many more cases of course and the medical journals are not going to publish every case that happens
  • An authoritative review from the European Centre for Disease Prevention and Control at the end of September could only find 6 cases they were convinced represented a second infection.  The countries they are from are shown below
  • Thus, whatever the theoretical risk of a second infection, this far the rates are incredibly low
  • I need to mention, as this links to my conclusions at the end of this post, that all the 4 of the 6 cases that were symptomatic nonetheless had relatively mild disease first time round

What about other corona viruses?

  • It is easy to forget that we have information about the risk of second infection from other corona viruses
  • A Dutch study published 4 weeks ago followed up 10 healthy male volunteers from 1989 to now and examined how often they had antibody evidence of an infection with one of 4 seasonal coronaviruses
  • Based on the antibody tests re-infection with one or more of the viruses was seen in all the men
  • There are also some data from the first SARS epidemic from the laboratory records of over 130,000 swab positive cases in Qatar.   In that study the rate of re-infection was just 4/10000 during an interval between 45-129 days. The laboratory records may have missed other people with re-infection
  • Honestly I am not sure these conflicting data help much!

How do we know for certain that a second infection is in fact a new infection?

  • This is an important question (which is why I raised it!)
  • In reports both in the scientific press and the media, different approaches have been used to suggest a second infection.  Typically, they have been based on a second positive swab test, at some interval after a first positive test
  • The reported cases vary as to whether they were symptomatic at one or both time points (see below for the six ‘robustly identified cases” in the ECDC report
  • There are a number of reasons other than a new infection that might explain a second positive swab test.  These include:
    • The swab (PCR) test being false positive on one or more occasions (rare but recognised as a possibility)
    • Some people continue to shed virus for up to 100 days after their infection even though they are clinically well
    • A related point is that when a second clinically obvious infection occurs very shortly after a first, it is not certain that it is not just (a rare) continuation of the first 
    • A PCR test shows evidence of viral RNA (a genetic footprint that the virus was there), it does not prove that the virus is still there in active amounts to cause infection 
  • The only evidence that an infection is really new comes from a detailed analysis of the genetic structures of the two positive swab tests
  • In the six cases above there was such evidence that the virus forms were different (or different mutations)

Is there a unifying story that can explain all the above?

I think so and this is my take (and in part this repeats some of my previous conclusions):

  1. Most people produce an antibody response to the virus, the level of this antibody response is greater, the more severe the disease
  2. In everyone the antibody response goes down over time but  this does not mean that we are not protected against further infection
  3. The hope is that the body’s immune mechanisms will protect against further infection with either the same strain of the virus or with minor mutations
  4. Within the current knowledge, ie basically just 6 months, this is all still true
  5. It would not be surprising with increasing passage of time, especially in those with mild or asymptomatic infection first time round, that the antibody response may not prove to be sufficient to protect against a new infection with a similar or quite different virus strain
  6. It is also true thus far that second infections have not been associated with a severe outcome 

And in terms of future actions and behaviours..

  • It makes sense that whatever evidence there is of having had a previous infection, following the current prevention advice is still appropriate 
  • Yes of course the question is raised again what does this imply about the future success of a vaccine? No one knows but I can only repeat that my understanding is that targeting the spike of the virus, however it might mutate, may still prove to be effective
  • I’m leaving my scientific self here and wanted to finish by saying please do not be depressed by all this!  In many ways none of this is new to virologists and infectious disease epidemiologists.  With all the cautions I can muster, I do not believe that there is a significant risk that the population will be left continually susceptible to new strains of this microbe because of our imperfect immunity  

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3 replies on “What is the risk of catching Covid-19 twice?”

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