Researchers in Spain gave details of the healthcare worker, who tested positive a few days before Christmas in December 2021 and again in January 2022. The case is further evidence that the Omicron variant can evade immunity from even recent previous infections.
The woman, who was fully vaccinated and had received a booster shot 12 days earlier, tested positive in a PCR staff screening test at work on 20 December. She didn’t develop any symptoms, and self-isolated for 10 days before returning to work.
On 10 January 2022, just 20 days after first testing positive, she developed a cough, fever and felt generally unwell and did another PCR test. This was also positive.
Whole genome sequencing showed that the patient had been infected by two different Covid variants. Her first infection was with the Delta variant and the second was with the Omicron variant, 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 case underlined the importance of genomic surveillance. “Such monitoring will help detect variants with the ability to partially evade the immune response,” she said.
Reinfections are recorded in the UK, but require 90 days between positive tests. Official figures suggest that nearly 900,000 people in England had potentially been infected twice with Covid up to the start of April. However, the number is not exact because only whole-genome sequencing can pinpoint whether infections are caused by different variants, and not all infections are reported.
Paul Hunter, a professor in medicine at the University of East Anglia, said: “This case is not particularly surprising, though the gap between infections is particularly short. We have known for some months that reinfections will occur. The Omicron variant with its escape mutations has made reinfections even more likely.”
Now that Omicron is the dominant variant, it is possible that prior infection with Omicron will make reinfection – especially so quickly – less likely. Previously scientists had predicted that as Covid-19 moves into an endemic phase, reinfections are likely to occur within a range of three months to five years.
“We can expect further waves of infection especially during winter even without new variants,” said Hunter. “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.”
Prof Lawrence Young, a virologist at the University of Warwick, said: “While it is difficult to extrapolate from a single case, this report highlights the ability of the Omicron variant, and its sub-variants, to reinfect even in those individuals who are fully vaccinated … This accounts for the extremely high levels of infection we have experienced in the UK.”