Nosocomial COVID-19 Transmission Often Detected Within 5 Days of Exposure

Most patients with nosocomial COVID-19 infection tested positive within 5 days of exposure, and mortality risk was lower for those infected with Omicron vs prior variants.

Nosocomial COVID-19 transmission was found to most commonly occur within 5 days of initial exposure, and the risk of mortality associated with infection with the Omicron variant was lower compared with previous variants. These study results were published in the Journal of Hospital Infection.

Researchers conducted a retrospective study among patients hospitalized at a single center to determine rates of nosocomial COVID-19 transmission following potential exposure events. The risk for nosocomial COVID-19 transmission was evaluated during the periods of Alpha (January-March and April-July 2021), Delta (August-October 2021), and Omicron (November 2021-January 2022) predominance. The researchers calculated the secondary attack rate, defined as the number of patients with confirmed infection divided by the number of patients for whom the outcome of a potential exposure event was known. The risk of 30-day mortality was assessed among all exposed patients who tested positive for infection, and mortality rates for variant-specific period of predominance were compared.

Univariable mixed-effects logistic regression was used to determine associations between COVID-19 transmission and index case transmission attribution, hospital location at time of exposure, and date of exposure. Patients with index infection were inpatients with polymerase chain reaction-confirmed infection who were in close proximity to patients considered to be susceptible to infection.

The analysis included a total of 1378 patients (median age, 70 [IQR, 52-81]; 57% men) who were potentially exposed to a patient (n=346; median age, 68 [IQR, 68-81]; 55.0% men) with index infection. Of patients with potential exposure, the majority (94%) were exposed to 1 index case, and 46% were unvaccinated.

Ongoing surveillance in hospitals will greatly help track vaccine effectiveness in reducing transmissibility and mortality of patients with nosocomial SARS-CoV-2 infection…

Of 1291 exposed patients who were available for SARS-CoV-2 testing, 87 tested positive for infection. Analysis of these patients indicated a secondary attack rate of 15.5% (95% CI, 13.5%-17.5%), and older patients were more likely to test positive compared with younger patients (median age, 76 vs 68 years, respectively; P <.001).

The researchers found exposure risk was significantly associated with hospital location. Compared with patients exposed in admission wards, the risk for COVID-19 transmission was 3.0- (95% CI, 2.9-20.8), 2.8- (95% CI, 1.3-5.9), and 7.8-times (95% CI, 2.9-20.8) higher for those exposed in medical, surgical, and rehabilitation wards, respectively. Patients who developed COVID-19 infection had a median exposure duration of 3 (IQR, 1-6) days, with most testing positive at days 4 (75%) and 5 (80%).

Compared with patients who tested positive for infection between January and March 2021, the risk of mortality was significantly lower among those who tested positive during the period in which the Omicron variant was predominant (odds ratio, 0.32; 95% CI, 0.10-0.92; P =0.04).

Study limitations include the observational design. Moreover, the rate of nosocomial COVID-19 infection may have been underestimated as some patients were not screened following exposure.

According to the researchers, “Ongoing surveillance in hospitals will greatly help track vaccine effectiveness in reducing transmissibility and mortality of patients with nosocomial SARS-CoV-2 infection…”


Hawkins LPA, Pallet SJC, Mazzella A, et al. Transmission dynamics and associated mortality of nosocomial COVID-19 throughout 2021: a retrospective study at a large teaching hospital in London. J Hosp Infect. Published January 9, 2022. doi:10.1016/j.jhin.2022.12.014