After the first Omicron wave, many of the risks of Covid infection changed. Before February 2022, wearing face masks and being over 70 were credited with a reduced risk of being infected by others – but not after that, according to a new study at the University of East Anglia (UEA) in Norwich, the UK.
The analysis of official data found that several risk factors for infection altered significantly as the dominant variant in the UK and most countries around the world changed from Delta to Omicron in December 2021. These included wearing a mask, a history of foreign travel, household size, whether people were working or retired, and contact with children or people over the age of 70. In addition, traveling abroad was not associated with increased risk prior to February 2022 but then became a significant risk.
Lead author Prof. Paul Hunter of Norwich Medical School said: “Early in the pandemic, there were many studies published looking at risk factors for catching the new Coronavirus, but far fewer studies after the first year or so. Our research shows that there were changes in some risk factors around the time that the Omicron BA.2 variant became dominant.”
'Not particularly surprising'
Co-author Dr. Julii Brainard said: “This isn’t totally surprising because laboratory evidence suggests that the Omicron variant was better able to infect the cells lining the upper respiratory tract than previous variants, and so be more transmissible. “Management of infection risk needs to be agile, adapting to epidemic development and better-quality information when it emerges. To prevent infections, we need to have a good view of which factors might be most or least relevant. If those factors can change, we need to be alert to that happening.”
The team published the study in the journal PLOS titled “Changing risk factors for developing SARS-CoV-2 infection from Delta to Omicron.”
The researchers analyzed data available from England’s Office for National Statistics (ONS) COVID survey, which compared infection rates with an ongoing household survey of the population to estimate the number of people with infections.
From November 2021 to May 2022, the ONS also asked people questions about their circumstances and habits to see if those factors could be linked to the risk of positivity. “We used this dataset to look for constancy or change in the importance and direction of potential risk factors for testing positive. We applied a statistical method called meta-regression to do this,” Hunter added.
In November 2021, always wearing face masks at work, school, or in enclosed spaces was connected with a reduced risk of being infected in both adults and children, but after the first Omicron wave, it was not. Living in a house with five or more people was a risk at the beginning, but by the end of the study period, people in larger households (four and above) had negligibly greater risk than people living in single-person households.
Early overseas travel was not associated with increased risk, but later on, it was. Working in health or social care or in contact with others was often found to be important in the first year of the pandemic but was not associated with an overall higher or changing risk of infection in the study period. Being a member of an ethnic minority was strongly associated with increased risk in the first few months of the UK epidemic. Still, it posed a lower risk and no significant trend change during the study’s full monitoring period.
Being retired was associated with reduced risk compared to those in work overall, but any protective effect had disappeared by February 27, 2022, which coincided with the start of the second Omicron wave. By the end of February 2022, it became apparent that there was a decrease in risk for adults living with children aged 16 or under. People under 70 who lived with someone aged 70 or older initially had a lower likelihood of testing positive, but this protective effect diminished by about mid-February 2022.
The researchers said the balance of evidence shows that wearing face coverings reduce transmission of respiratory infections in community settings and reduce transmission of the virus, but the question is by how much.
Systematic review of pre-pandemic evidence and analysis of original survey data during the pandemic both indicated that mask-wearing could or did reduce transmission by about 19%, but these conclusions were derived mainly from data prior to the emergence of Omicron variants.
This latest research found that prior to Omicron BA.2, never wearing a mask was associated with an increased risk of around 30% in adults and 10% in children. However, by the second Omicron wave (mid-to-late February 2022 onwards), mask-wearing had no protective effect on adults and possibly increased the risk of infection in children.
“It should not be a surprise that risk factors change during a pandemic due to a highly infectious disease with a short duration of immunity like COVID-10,” Hunter said. “We offer some possible explanations for why the changes may have happened, but we would need more focused research to understand for sure why there were changes in some risk factors,” Brainard concluded.