Severe disease from Covid reinfections very rare, study suggests

Reinfections of Covid-19 are 90 per cent less likely to result in hospitalisation or death than primary infection, a new study suggests.

Scientists from Qatar found that it is rare for people to be reinfected with the virus. And when an individual does catch Covid for a second time, the disease is generally mild in nature, the researchers said.

Out of 353,326 people who tested positive in Qatar between February 2020 and April 2021, a total of 1,304 reinfections were identified. The analysis excluded 87,547 people who were vaccinated.

Just four of the reinfections were severe enough to lead to acute care hospitalisation. None required ICU admission, nor did any end in death. Among the primary infections, 28 people were admitted to an ICU, while seven died.

The study, published in the New England Journal of Medicine, concluded that reinfections were not only milder than the initial infection, but far less likely to lead to hospitalisation or death.

Reinfection was defined as the first positive swab obtained at least 90 days after being initially diagnosed with Covid-19.

The vast majority of the reinfections were caused by the Beta variant, which is more effective than other mutations at evading the body’s immune response.

Researchers also found that the median time between the first and second infection was around nine months.

The first wave of infections in Qatar struck between March and June of 2020, after which 40 per cent of the population had detectable antibodies against Covid-19.

The country then experienced back-to-back waves from January through to May of 2021, which were driven by the Alpha and Beta variants.

On the basis of their results, the scientists suggested that Covid “could adopt a more benign pattern of infection when it becomes endemic”.

However, they warned, “it needs to be determined whether … protection against severe disease at reinfection lasts for a longer period, analogous to the immunity that develops against other seasonal ‘common-cold’ coronaviruses, which elicit short-term immunity against mild reinfection but longer-term immunity against more severe illness with reinfection”.

David Matthews, a professor of virology at the University of Bristol, who was not involved in the study, said the findings suggest that Covid will “eventually become like the common cold” as more people get vaccinated and exposed to the virus, before then recovering.

“Your immune system can rely on the recent memory of previous victories in order to defeat the virus when it sees it again.

“You can't be certain, but what probably will happen is is that this virus will become a common cold virus, like the human coronaviruses that are already common.

“The findings also re-emphasise that the pressure on us right now in terms of hospitalisations is primarily coming from people, who for whatever reason, still haven't been vaccinated.”

Paul Hunter, a professor in medicine at the University of East Anglia, said the paper supports pre-existing evidence which shows that “reinfections are usually but not always less severe than initial infections”.

However, he suggested that it was difficult to fully determine the severity of reinfection. “It is often difficult to judge this properly,” he said. “If you are basing your assessment on people turning up in hospital, then generally people go to hospital when they get quite sick.

“Reinfections may look of similar severity but that is because people with less severe illness [who have been reinfected] don’t go to hospital.”

Prof Hunter also said that “natural infection does seem to give more durable immunity than vaccine, and probably better cross protection against new variants, but that depends on how severe the initial infection”.