Effect of Nonpharmaceutical Interventions on COVID-19 Transmission Risk After High-Risk Exposure

Virus cells. Coronavirus Covid-19 concept image.
Researchers conducted a study to assess whether nonpharmaceutical interventions effectively prevent COVID-19 transmission following high-risk exposures.

The use of nonpharmaceutical interventions (NPI) was found to effectively decrease the risk for COVID-19 transmission in patients exposed to an individual with confirmed or suspected COVID-19 infection, according to results of a study published in Clinical Infectious Diseases.

Researchers conducted a test-negative design case-control study among patients in California who underwent SARS-CoV-2 testing between February and November 2021 following a high-risk exposure; all results were reported to the California Department of Public Health. A high-risk exposure was defined as social contact with an individual with confirmed or suspected COVID-19 infection within the past 14 days. The study comprised a total of 2891 patients, of whom 1448 were positive (cases) and 1443 (controls) were negative for COVID-19 infection. Eligible patients included those who provided informed consent and had no previous diagnosis or positive test result for COVID-19 infection. The researchers sought to assess whether NPIs decrease the risk for COVID-19 transmission following a high-risk exposure. The researchers used conditional logistic regression to estimate the adjusted odds of COVID-19 transmission among patients who reported a high-risk exposure.

Among patients included in the study, 54.3% were women, 45.7% were White, 35% were aged 30 to 49 years; and 68.4%, 8.7%, and 22.9% were unvaccinated, partially vaccinated, or fully vaccinated against COVID-19, respectively. Among patients included in the case and control groups, 751 (52%) and 255 (18%) reported a confirmed or suspected high-risk exposure within the 14 days prior to undergoing SARS-CoV-2 testing, respectively. Of patients (n=847) who had a high-risk exposure, 55% and 14% reported the exposure occurred within their household and workplace, respectively.  

The researchers found that the risk for COVID-19 transmission following a high-risk exposure was increased among patients who reported the exposure occurred with a household member (aOR, 3.02; 95% CI, 1.75-5.22). Further analysis showed an increased risk for COVID-19 transmission among patients whose high-risk exposures occurred indoors vs outdoors (aOR, 2.10; 95% CI, 1.05-4.21). For both unvaccinated and vaccinated patients, the risk for COVID-19 transmission was increased when exposure duration was 3 hours or more; however, this increased risk was mitigated among fully vaccinated patients.

The researchers noted that 14% of patients who tested positive for COVID-19 and 34% of those who tested negative reported mask usage during high-risk exposures (aOR, 0.50; 95% CI, 0.29-0.85). Of note, the use of masks was found to effectively protect against COVID-19 transmission among patients whose high-risk exposures occurred either outside their household (aOR, 0.39; 95% CI, 0.22-0.79), without physical contact (aOR, 0.37; 95% CI, 0.20-0.69), or indoors (aOR, 0.51; 95% CI, 0.28-0.93).

This study was potentially limited by exposure misclassification due its reliance on patients’ self-reported behaviors during high-risk exposures, the lack of information on high-risk contacts’ infection status, and the possibility that COVID-19 transmission occurred via interactions other than the high-risk exposure event.

According to the researchers, “these findings may inform the use of NPIs in populations with limited vaccine access or those ineligible to be vaccinated, and in response to changing epidemiologic conditions such as [the] emergence of [SARS-CoV-2] variants associated with [increased] infectiousness.”

Disclosure: Some authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of disclosures.


Andrejko KL, Pry J, Myers JF, et al. Predictors of SARS-2-CoV infection following high-risk exposure. Clin Infect Dis. 2021; doi:10.1093/cid/ciab1040

This article originally appeared on Infectious Disease Advisor