Transmission dynamics and associated mortality of nosocomial COVID-19 throughout 2021: a retrospective study at a large teaching hospital in London.

Background: The impact of nosocomial SARS-CoV-2 infections has changed significantly since 2020. However, there is a lack of up-to-date evidence of the epidemiology of these infections which is essential in order to appropriately guide infection control policy.

Aims: To identify the secondary attack rate of SARS-CoV-2 infection and associated mortality across different variants of concern.

Methods: A single-centre retrospective study of all nosocomial SARS-CoV-2 exposure events was conducted between 31 December 2020 and 31 December 2021. A secondary attack rate was calculated for nosocomial acquisition of SARS-CoV-2 infection and time to positivity. Positive contacts were assessed for all-cause 30-day mortality.

Results: A total of 346 sequential index exposure events were examined, and 1378 susceptible contacts identified. Two hundred susceptible contacts developed SARS-CoV-2 infection (secondary attack rate of 15.5%). The majority of index cases (59%) did not result in any secondary SARS-CoV-2 infection. Where close contacts developed SARS-CoV-2 infection, 80% were detected within the first five days since last contact with the index case. The overall associated mortality among positive contacts across 2021 was 9%, with an estimated reduction of 68% when comparing periods of high Omicron versus Alpha transmission.

Conclusion: Our findings describe that most SARS-CoV-2 infections are detected within five days of contact with an index case; we have also demonstrated a considerably lower mortality rate with the Omicron variant in comparison to previous variants. These findings have important implications for informing and supporting infection control protocols to allow movement through the hospital, and ensure patients access care safely.