How quickly can you catch Covid again?

A 31-year-old Spanish woman has caught Covid twice within 20 days, the shortest documented gap between infections, scientists have said.

The healthcare worker became infected with the Delta variant followed by the Omicron strain of the virus in under three weeks. Researchers say the case shows that even vaccinated people who have had Covid-19 recently “cannot assume they are protected against reinfection”.

The woman, whose name has not been released publicly, was “fully vaccinated and had received a booster shot 12 days earlier”, reported The Guardian. She tested positive in a PCR staff screening test at work on 20 December 2021. She “didn’t develop any symptoms, and self-isolated for 10 days before returning to work”.

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Then, on 10 January 2022, she “developed a cough, fever and felt generally unwell and did another PCR test”, which came back positive again.

Whole genome sequencing showed that the patient had been “infected by two different Covid variants”, said the paper. The healthcare worker had first been infected with the Delta variant before then contracting the Omicron virus, “which is known to be more infectious and can evade immunity from past infections and vaccination”.

Dr Gemma Recio, of the Institut Català de la Salut in Tarragona and one of the study’s authors, said: “This case highlights the potential of the Omicron variant to evade the previous immunity acquired either from a natural infection with other variants or from vaccines.”

She added that the use of genomic sequencing “will help detect variants with the ability to partially evade the immune response”.

The case, which is set to be presented to the European Congress of Clinical Microbiology and Infectious Diseases in Portugal, is “believed to be the shortest known gap between infections”, said Sky News.

How soon can you be reinfected with Covid-19?

Previously in the UK, if you tested positive for any variant of Covid-19 within a 90-day time period it was “considered to be part of the same case episode”, said the i news site. Testing positive weeks after an initial positive test was likely to be from the “residual effects of the initial infection”, while tests outside the 90-day period were counted as a reinfection.

But with Omicron now the dominant variant, those who have been infected with other variants like Alpha, Beta and Delta are at risk of reinfection because Omicron is better at evading immunity from past infections. A study published in December by the World Health Organisation (WHO) and Imperial College London found Omicron was “five times more likely to reinfect people than Delta”.

However, it is “possible” that prior infection with Omicron will make reinfection less likely, “especially so quickly” after an initial infection, said The Guardian.

“We can expect further waves of infection especially during winter even without new variants,” Paul Hunter, a professor in medicine at the University of East Anglia, told the paper.

“Fortunately the evidence is that immunity to severe disease is more robust than immunity to infection. So even though reinfections will continue to occur for many years, we will see fewer and fewer severe illnesses and deaths with time.”