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Young people need the COVID vaccine even if they've been infected before © Getty Images

Young people need the COVID vaccine even if they've been infected before

Published: 16th April, 2021 at 10:10
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A study of young men suggests that previous infection does not fully protect against reinfection.

Previous coronavirus infection does not fully protect young people against reinfection, research suggests.


Researchers said that despite previous infection and the presence of antibodies, vaccination is still necessary to boost immune responses, prevent reinfection and reduce transmission. They added that young people should take up the vaccine whenever possible.

According to an observational study of more than 3,000 healthy members of the US Marine Corps, most of whom were aged 18-20, between May and November 2020, around 10 per cent (19 out of 189) of participants who had previously been infected with SARS-CoV-2 (seropositive) became reinfected. This was compared with new infections in 50 per cent (1,079 out of 2,247) of participants who had not previously been infected (seronegative).

Although the study was in young, fit, mostly male recruits, the researchers believe the risk of reinfection will apply to many young people.

However, the exact rates of reinfection will not be applicable in other settings, because of the crowded living conditions on a military base and close personal contact required for basic training likely to contribute to a higher overall infection rate than elsewhere.

For example, a study of four million people in Denmark found that the risk of infection was five times higher in people who had not had the disease before. But researchers found that only 0.65 per cent of those who had had COVID-19 during Denmark’s first wave tested positive again during the second wave, compared with 3.3 per cent of people who tested positive after initially being negative.

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In addition, a preprint study including British healthcare workers found that those who had not been previously infected had a five times higher risk of being infected than people who had a past infection.

“As vaccine rollouts continue to gain momentum it is important to remember that, despite a prior COVID-19 infection, young people can catch the virus again and may still transmit it to others," said Professor Stuart Sealfon, of Icahn School of Medicine at Mount Sinai, New York, and senior author of the study.

“Immunity is not guaranteed by past infection, and vaccinations that provide additional protection are still needed for those who have had COVID-19.”

Recruits who tested positive for a second infection during the study were isolated and researchers followed up with additional testing. Levels of neutralising antibodies were also taken from subsequently infected seropositive and selected seropositive participants who were not reinfected during the study period.

Of the 2,346 participants followed long enough for this analysis, 189 were seropositive and 2,247 were seronegative at the start of the study.

Across both groups, there were 1,098 (45 per cent) new infections during the study. Among the seropositive participants, 19 (10 per cent) tested positive for a second infection during the study. Of the recruits who were seronegative, 1,079 (48 per cent) became infected.

Graphic showing global COVID-19 cases and deaths as of 15 April © PA Graphics
© PA Graphics

The authors studied reinfected and not infected participants’ antibody responses to understand why these reinfections occurred. They found that, among the seropositive group, participants who became reinfected had lower antibody levels against the virus than those who did not.

Comparing new infections between seropositive and seronegative participants, the authors found that viral load – the amount of measurable virus – in reinfected seropositive recruits was on average only 10 times lower than in infected seronegative participants.

This suggests some reinfected individuals could still be able to transmit the virus, but the authors noted this will need further investigation.

In the study, most new cases were asymptomatic – 84 per cent (16 out of 19 participants) in the seropositive group, and 68 per cent (732 out of 1,079) in the seronegative group – or had mild symptoms, and none were admitted to hospital.

The authors noted some limitations to the study, including that they are likely to have underestimated the risk of reinfection in previously infected individuals because the study does not account for people with very low antibody levels following their past infection.

They were also unable to determine how seropositive recruits contracted their previous infection and confirm it by PCR test or determine how severe it was and what symptoms they had.


Researchers said they also could have missed detectable infections that occurred between the PCR testing every two weeks during the study, published in The Lancet Respiratory Medicine journal.

Reader Q&A: Will social distancing measures cause coronavirus to evolve into a more deadly strain?

Asked by: Gary Theobald

Viruses, like COVID-19, evolve rapidly. Each time the virus replicates, mutations can occur in its genome. Most of these mutations have no effect, or are even damaging to the virus. However, occasionally a mutation will arise that is advantageous for the virus. These mutations may allow the virus to grow faster, spread better or evade our immune system. The longer a virus continues to circulate, the greater the chance of these mutations occurring and the virus evolving into a new strain that behaves differently.

However, causing more severe disease isn’t necessarily advantageous to the virus. One of the reasons the COVID-19 virus is so difficult to contain is that it spreads very well before people become sick. If the virus spreads while causing more severe symptoms, people with COVID-19 would likely stay home instead of going out (reducing transmission) and would seek medical attention (enabling more effective testing and contact tracing). Sometimes mild viruses are the most difficult to eliminate.

Because the COVID-19 pandemic has been so difficult to contain, we have all had to take precautions, such as social distancing, washing hands and wearing masks. Each of these individual precautions limits the amount of virus that can spread from one person to another. These actions could put pressure on the virus to evolve, possibly resulting in mutated strains that are more transmissible and more difficult to control (though not necessarily deadlier). However, by combining all of these precautions, together with strong public health infrastructure (such as test and trace), we can still effectively stop the transmission of COVID-19, even if it evolves into a more virulent strain.

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.


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