Since the beginning of the pandemic – for both the public and policy makers – one question has been “Can someone catch Covid-19 twice?”: specifically, everyone wants to know not just “is this a possibility” but “what is the likelihood that this could happen?”. Further, if immunity following an infection does wane over time, and given that we are now 12 months into the pandemic, is the rate of cases of a second infection increasing? These questions are brought into recent focus with the spread of the new variant, with concern that immunity acquired from earlier variants may not now be protective. In this post, I summarise what we know.
Defining ‘catching Covid-19 twice’
- There are some challenges in proving that someone had caught the infection twice
- Broadly speaking it is highly unlikely that someone would continue to be shedding virus (posh word for spreading active virus from nose and mouth), more than one month after the start of an infection
- Thus clinically, when a person has been ill, recovered and is symptom free, a new infection after 28 days is broadly considered to be a second infection
- This would require a new positive swab test to confirm
- There are however several reports of people still being positive on swab tests up to 8 weeks after their first positive test
- For that reason, ‘proof’ of a new infection requires detailed genetic analysis of the virus between the two positive swabs to show that they are different.
- To complicate matters a little (!) it is not impossible for someone to have two separate strains causing their infection at the same time – but this is probably highly unusual
- However common sense would suggest that a person who had an obvious infection, recovered well and then some months later developed a second diagnosed infection, had caught the infection twice
“I have had one infection, I want to know my risk of having a second infection”
There are two ways to approach this question:
- The direct way; by providing information on the rate of second infection, over the chosen time period – which could be the next month, the next year or lifetime!
- The indirect way; by measuring the level of immunity by antibody tests for example, on the understanding that such knowledge would be an accurate guide to the future risk of a getting ill with an infection
- These are considered in turn below
What is the rate of second infections?
- Perhaps surprisingly we do not know how common is a second infection in people who have had one infection
- Thinking about this, the rate of second infection will be clearly related to how exposed a person is:
- If following a first infection the person stays at home and meets no one else, then their risk of a second infection would be almost zero
- Further although the rate of having an asymptomatic infection is the same across age groups, the risk of being ill increases with age. Thus, the rate of a second symptomatic infection will vary with the age and indeed other risk factors such as obesity and possibly ethnic background
- Perhaps for this reason there has not, as far as I can tell, been a comprehensive epidemiological study where people who have recovered have been regularly tested with a swab test to measure the rate of getting a further infection
- As mentioned above, to distinguish a new infection from a continuing old infection also requires sophisticated genetic analysis which is not widely available
- What does appear to be happening is that when a second infection does occur, the clinicians feel they should report such cases in the scientific press
- It is difficult to draw conclusions from what are at most a few handfuls of cases. Reports from the majority of individual patients, though not all, suggest the second infection is milder
- An informal report attempting to identify all cases worldwide (subject to masses of errors) had found 1673 by December from the several millions who we know had been ill
- A recent study from Oxford undertook an antibody test on 12000 health workers, of whom 1246 were positive for antibodies against Covid-19
- They were then followed up for over 6 months with swab tests: finding 3 with a positive result (all of whom were well)
- Conclusion: 12 months into the pandemic, second cases appear to be exceptionally rare
Do we know how long immunity last for?
- As we don’t know how common a second infection is, then we could get some idea if we knew how long the level of immunity following an infection lasts for
- One approach is to measure the levels of antibodies in the blood at different intervals after an infection
- This has been done in several studies, but of course we only have data for the length of time that people have been followed up, which – given the global pandemic only started in spring 2020 – is not much more than 6 months
- Thus far though antibody levels in general do persist for 6 months
- The actual levels do decline over time, but this is normal after any infection
- What is true for Covid-19 is that antibody levels are lower, and may indeed by undetectable, in people who have had very mild disease or were asymptomatic
- Although that could mean that a mild dose of the disease does not confer immunity, it is also possible that a mild disease means that there was an efficient immune response to prevent a serious infection, that could come into play next time
- However just because the levels of antibodies decline does not mean that there will not be an adequate immune response if faced with the virus again
- The body’s immune system, following an infection with Covid-19, is primed to respond to a second infection in two ways:
- Producing antibodies that neutralise the virus
- Bringing some special cells, T cells, into operation that also can destroy the virus
- These responses are stored in our immune memory to be brought out of ‘hibernation’ when there is a signal of a second attack
- We still do not know how far the antibody memory and the T cell response would come into play if antibody levels were low or even undetectable
What about the new variant?
- If the virus causing the new infection is sufficiently different from that causing the first infection then the pre-existing immune response theoretically may not provide protection
- There has been one case reported last week of a person who contracted new variant Covid-19 (‘VOC – variant of concern’) months after recovering from a first Covid-19 infection, but one case does not prove any significant risk for all
- At the moment in truth there is inadequate research on whether for most people antibodies from a first infection will protect against infection with the new variant
- I am however reassured that thus far testing of the new vaccines suggest they should still protect against the new variant
- Remember the current vaccines work by stimulating the body’s response to just one part of the virus – the spike protein
- Natural infection with Covid-19 should produce antibodies to other parts of the virus, for example there is another protein called the N (nucleocapsid) protein.
- Indeed, this has also been targeted for vaccine development
- Thus the point being that pre-existing immunity from ‘old infection’ has a number of targets in any new variant that could still work
(picture credit https://news.ucsc.edu/2020/02/coronavirus-genome.html)
And Covid-19 is not the only coronavirus in history!
- There are some reassuring data from longevity of immune responses from other coronaviruses
- Coronaviruses that cause the common cold produce antibodies that last at least a year
- Further when people are re-challenged with the same virus some get infected again but the symptoms are milder
- People infected in the first SARS epidemic in 2003 still had a T cell immunity to that coronavirus this year, after 17 years – interestingly that was directed against the N protein
- And (not a coronavirus) but in 2008, 90 years after the Spanish flu pandemic, some people still alive had neutralising antibodies to that virus!!
- Yes of course it is reasonable to want to know about the risk of getting Covid-19 twice
- Thus far second infections seem to be exceptionally rare
- When they do occur, they tend to be mild
- The immune response following natural infection is also sustained for at least 6 months and protection may last longer
- We await further research, but at the moment no need to panic that the new variant poses a particular risk of a second infection
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