Coronavirus reinfection: what we know so far

Update from experts on what COVID-19 research suggests about prevalence and illness severity second time around

• Data is limited but the knowledge base is expanding all the time – find out what the science says now
• Patients can be reassured that reinfection appears uncommon, however it might still have a role in virus transmission
• Your questions answered on risk to nursing staff, illness severity and likelihood of contracting coronavirus twice

There are still many gaps in our understanding of COVID-19 and clinicians’ ability to treat it, but knowledge is advancing rapidly.

Now, researchers are adding to what we know about one aspect of the pandemic that has been overshadowed by the race for a vaccine: reinfection.

Here, we look at the evidence accumulated so far about coronavirus reinfection and whether nursing staff are at increased risk.

How many confirmed cases of reinfection have there been?

In short, not many, but that may reflect a lack of data rather than indisputable evidence.

A BMJ article published in January put the number of reinfections worldwide at 31, while a US-based reinfection tracker reported 57 confirmed reinfections globally by 25 February.

Those 57 cases had resulted in two deaths.

But delays in reporting and other pressures arising from the pandemic mean there is some uncertainty about the numbers and they may be higher.

Either way, reinfection is uncommon relative to confirmed cases of primary infection.

‘If you believe you already had the disease, you can be reassured it is highly unlikely you will develop severe infection, but there is still a risk you could acquire an infection and transmit to others’

Susan Hopkins, senior medical adviser, Public Health England

In a study by microbiologists at St George’s University Hospitals NHS Foundation Trust there were eight coronavirus reinfections among 10,000 patients.

The researchers say that because this is a new disease and data are therefore limited, it is possible that reinfections will become more likely as immunity decreases over time in those who have already had COVID-19.

However, alongside this more than 24 million people had received at least one dose of the COVID-19 vaccine by the start of the third week in March 2021 – protection against the virus ranges from 62% to 95% depending on the vaccine administered and the individual receiving it.

Research by Public Health England (PHE), which is known as the SIREN study and involves thousands of healthcare workers, including nurses, found 44 potential reinfections (two ‘probable’, 42 ‘possible’) and 409 new infections.

Previous infection offered 83% protection against reinfection for five months, the study found.

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So most people who have had the virus once are unlikely to get it again?

Yes, but there are two important caveats, says PHE senior medical adviser Susan Hopkins, who led the SIREN study.

First, it is not clear exactly how long protection lasts after an initial infection. Evidence from SIREN suggests it could be at least five months from a person first becoming ill, while researchers at the University of Oxford and Oxford University Hospitals NHS Foundation Trust found protection lasted at least six months.

But whether immunity lasts five months, six months or longer, people who contracted COVID-19 in the first wave may by now be vulnerable to catching it again.

The second caveat is that those who are infected for a second time may still be able to pass on the virus even if they have only mild symptoms or none at all.

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‘If you believe you already had the disease, you can be reassured it is highly unlikely you will develop severe infection, but there is still a risk you could acquire an infection and transmit to others,’ says Professor Hopkins.

The first reported case of anyone contracting the disease for a second time came in August last year.

A man from Hong Kong, who tested positive in March 2020 with mild symptoms, tested positive again four months later after a trip to Spain. He was asymptomatic the second time but had a high viral load.

Does reinfection lead to more or less severe illness?

Reinfection could in theory cause worse symptoms, but this is unlikely because even partial immunity would lead to a milder illness, says consultant medical microbiologist Aodhán Breathnach, lead researcher for the St George’s study.

Dr Breathnach explains: ‘We’ve seen no evidence of more severe illness in the limited number of reinfections we have observed, and some were completely asymptomatic.’

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But a mild first COVID-19 infection may mean a greater risk of reinfection because no immune response was developed the first time.

A UK case study in Clinical Medicine Journal describes how a 25-year-old doctor was infected twice with COVID-19 last year.

The first time, in April, his predominant symptoms were headache, fever and fatigue. The second episode, in October, was symptomatically milder, with a faster recovery.

Could ‘reinfection’ in fact be re-emergence of the original infection?

It’s possible.

Dr Breathnach says there are rare reports of COVID-19 infection lasting many months but these are usually in people who are severely immunocompromised.

‘We believe the cases we described in the St George’s study were true reinfections as they all occurred many months after the first infections, during the time of the second wave when a new strain of virus was circulating.’

‘It is possible some new strains will evade our immune responses, as happens with influenza’

Aodhán Breathnach, lead researcher, St George’s University Hospitals NHS Foundation Trust coronavirus reinfection study

The BMJ says differentiating between reinfection and reactivation of existing coronavirus requires genetic sequencing of samples taken during a first episode of illness and again when reinfection is suspected.

But increasing the amount of sequencing is a ‘tall ask’ because of pandemic pressures on laboratories.

Even though the UK does more sequencing than most countries, only 5-10% of samples are sequenced, according to the BMJ article.

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What impact will new variants have on reinfection rates?

It’s hard to say.

New, more transmissible variants may lead to an increase in the total number of coronavirus cases, which in turn may increase the risk of reinfection.

‘We know from other coronaviruses that reinfections even with the same strain can occur within a year as immunity wanes,’ Dr Breathnach says.

‘It is possible that some new strains will evade our immune responses, as happens with influenza.’

Are nurses likely to be at greater risk of reinfection than the general population?

Dr Breathnach thinks not.

‘There’s no reason for them not to develop the same immune response as everyone else.

‘Obviously, working in front-line healthcare will put nurses and other health workers in contact with COVID patients but the combination of vaccination, personal protective equipment and careful hygiene will greatly reduce the risk, whether or not they have had prior infection.’

In fact, the low rate of reinfection among the healthcare workers in the SIREN study may not be typical of the wider population, according to the journal Nature.

It quotes immunologist George Kassiotis from the Francis Crick Institute, who notes that most of the participants in the SIREN study were women aged under 60, which may put them at lower risk of reinfection than other groups.

‘This group is unlikely to experience the most severe form of COVID-19 and may not be representative of the population as a whole,’ he says.

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When will we learn more about COVID-19 reinfection rates?

New studies are being undertaken all the time, so the body of evidence is growing, although long-term data are obviously lacking because COVID-19 is a new disease.

The PHE SIREN research will continue to follow study participants to explore how long any immunity against the virus may last.

It will also consider the effectiveness of COVID-19 vaccines and the extent to which people with immunity are still able to transmit the virus.

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Daniel Allen is a health journalist