What we know about Covid reinfection, immunity and vaccines

How long does natural protection from a first infection last?

There is no clearcut answer to this, but several studies suggest protection generated by a previous infection lasts for at least a few months.

According to one preprint study from Public Health England (PHE) released in January, which looked at hospital staff, the median interval between the first infection and reinfection was more than 160 days – about five months. Meanwhile, a study from Qatar suggests protection by natural immunity of about 95% efficacy lasts about seven months.

Is a reinfection likely to be worse or milder than a previous infection?

Again, that is unclear. “Different people will react differently to reinfection, depending on how their immune responses reacted to the first infection, probably,” says Julian Tang, a clinical virologist and honorary associate professor in the respiratory sciences department at the University of Leicester.

For some, a second infection is less severe than the first. According to a study from Qatar, less than 0.2% of people tested positive for Covid at least 45 days after their first positive test, with only about a fifth of these showing strong or good evidence for reinfection. Of these 54 people, just one was hospitalised, and even then only with a mild infection.

A second study from Qatar – yet to be peer-reviewed – supports this, with two-thirds of reinfections only picked up through random or routine testing. Again it suggest reinfection is rare, with just 129 people out of 43,044 followed showing evidence of reinfection over a median of 16.3 weeks.

The PHE study also suggested that reinfection tended to be less severe, with about a third of those who caught Covid for a second time showing symptoms, compared with 78% for first infection.

But there have been a number of cases around the world of reinfection leading to worse disease. A recent study from researchers in Brazil, about to be published in the Journal of Infection, found that of 33 people thought to have caught Covid for a second time, one died, and 12 were hospitalised – although none required such care for their first infection. However, it should be noted that it is not clear if all of the cases were reinfections or a flare-up of a previous infection.

Is it more likely that someone could catch Covid a second time from a different variant?

“If you didn’t have a good immune response, you could get infected again by exactly the same virus,” says Deborah Dunn-Walters, a professor of immunology at the University of Surrey and the chair of the British Society for Immunology’s Covid-19 and immunology taskforce.

If that immune response was good, the chances of being reinfected by the same variant will be lower, but reinfection might still occur by other variants.

However, the situation is not black and white as this depends on the mutations a new variant contains, and how they affect the ability of the virus to infect the cell and its interactions with the body’s antibodies and T-cell responses generated by the immune system as a result of the previous infection.

The possibility for a new variant to fuel reinfections has been highlighted by researchers in Brazil: despite about three-quarters of the population of Manaus thought from antibody tests to have been infected with Covid by October, there was a sharp uptick in hospital admissions for Covid in January this year. One explanation, they say, is the emergence of new variants of the coronavirus that may evade immunity gained from earlier infection.

Indeed, research published this week by researchers in Oxford, yet to be peer-reviewed, revealed that people who had recovered from Covid showed T-cell activity towards new variants, including the South African variant. But in general their antibodies were less able to neutralise the Kent and South African variant than the original coronavirus variant, suggesting a potentially lower level of defence.

Are some people more at risk of reinfection than others?

It appears so, but there are several factors at play. “Whether you catch it or not is a combination of whether you have got immunity and whether you have seen [the virus],” says Dunn-Walters.

Some people may be at greater risk because of social factors such as occupation, which means they have greater chance of coming into contact with the virus again – for example, healthcare workers would be expected to be at greater risk of both infection and reinfection because of this.

But there are also biological factors that might leave some people more at risk of catching Covid for a second time. “Each human is unique, as are their immune responses, which govern both the risk of reinfection and the severity of these reinfections, so it is very difficult to generalise research findings and clinical trial results to individuals in any population,” says Tang.

How important is vaccination?

Vaccination plays a key role in protecting individuals from a first infection. But it is also important for those who have already had Covid. While natural immunity can be gained from a previous infection, jabs give much more certainty over the level of protection produced and boost protection gained from a previous infection.

Vaccines may also offer greater protection against different variants. According to the preprint by Oxford researchers, people who received two doses of the Pfizer/BioNTech jab had a strong T-cell and antibody response against the original coronavirus and the Kent and South African variants, suggesting the vaccine probably offered protection against infection for all of these variants. That contrasts with the findings for those who had only previously had a natural infection.

“Natural infection doesn’t guarantee you immunity as well as perhaps the vaccination might,” says Dunn-Walters.

While studies have suggested that some other Covid vaccines may be less effective against the South African variant than against the original or Kent variants of the coronavirus, experts say these jabs still offer good levels of protection against serious disease. What’s more, vaccines are being tweaked to better target new variants, a move that will also bolster protection.