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CDC study suggests quarantine after potential Covid-19 exposure despite use of face masks

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Dr Surya Kant, Professor and Head, Dept. of Respiratory Medicine, KGMU, UP, Lucknow. National Vice Chairman IMA-AMS    15 October 2021

Quarantine after Covid-19 exposure is helpful even if the patient as well as the contact were wearing masks, suggests a new CDC study reported in the Emerging Infectious Diseases journal.

Data for the study was collected from October 20, 2020 to March 1, 2021. The study included 966 contacts associated with 431 patients with Covid-19. Out of these 966 contacts, 198 were positive for Covid-19, while 768 tested negative, resulting in an overall SAR of 20.5%. The effectiveness of masks for reducing SARS-CoV-2 transmission was determined by comparing the secondary attack rates (SAR) when both the case and the contact were wearing masks at the time of exposure vis à vis when one or more persons was not wearing a mask.

The number of contacts exposed when >1 person was not wearing a mask was 590 (61%), while 376 contacts (39%) were exposed when both the case and the contact were wearing masks at the time of exposure.

Out of the 590 contacts (when >1 person was not wearing a mask),  439 tested negative and 151 tested positive for Covid-19 resulting in an SAR of 25.6%. Out of the 376 contacts (when both the case and the contact were wearing masks), 329 tested negative and 47 tested positive with a SAR of 12.5%. The study found comparable SARs for unmasked (25.2%) and masked (12%) exposures among school-age children.

The SAR for contacts exposed when only the patient was masked was 29.1% compared to SAR of 10% when only the contact was masked suggesting that masks are more beneficial when worn by the contact than by the patient.

Duration of exposure was also found to affect the risk of transmission. The likelihood of a positive Covid test doubled with exposure for longer than 2 hours compared to exposure of less than 2 hours.

These results show that wearing masks reduced the SARs by half, suggesting that proper use of masks is very effective for reducing transmission of SARS-CoV-2. Expectedly, longer duration of exposure was associated with higher SARs. Although the study did not carry out a symptom-specific analysis, the authors suggest that transmission rates may be higher for persons with symptoms such as cough or fever than for those with symptoms such as headache and fatigue.

This study was conducted to examine whether the recommendation of the Iowa Department of Public Health, issued in September 2020, stating that “persons exposed to a Covid-19 patient need not quarantine if both of them were wearing face masks at the time of exposure and that symptom monitoring was appropriate”, could reduce transmission of the virus. On the other hand, the CDC recommended home quarantine after exposure to Covid-19, regardless of mask use. 

This study corroborates that while use of masks is protective, one must self-quarantine after contact with a person who has Covid-19, even if the patient and the contact were wearing masks.

Reference

  1. Riley J, et al. Mask effectiveness for preventing secondary cases of COVID-19, Johnson County, Iowa, USA. Emerg Infect Dis. 2021 Oct 12;28(1). doi: 10.3201/eid2801.211591. Online ahead of print.

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