Study population and setting
This study followed community health care workers who counseled asymptomatic family contacts of index SARS-CoV-2 cases in Chennai, India. Rates of laboratory-confirmed SARS-CoV-2 infection among health care workers were compared between two periods: from May 3 to May 15, 2020, before the introduction of 250-micrometer polyethylene terephthalate (PET) face shields (n=62); and from May 20 to June 30, 2020, after face shield introduction (n=50). Workers lived in separate rooms of a hostel and had no physical contact with one another after the training period; they did not visit their homes or public places during the study period. Workers traveled between households in vans with partitions separating them from drivers. Throughout the study period, workers also used surgical masks, gloves, shoe coverings, and alcohol-based hand sanitizers. During household visits, masked workers provided SARS-CoV-2 counseling in the front room of households, standing at least 6 feet away from assembled family members. Family members were advised to wear masks, although some did not wear them. On May 20, 2020, face shields were added to protocols; shields were treated with alcohol sanitizer after each visit and soaked in a water-detergent solution nightly.
Summary of Main Findings
In the pre-shield period, 62 workers visited 5,880 households with 31,164 people; 222 people in these households later tested positive for SARS-CoV-2. Two workers developed symptoms (symptoms appeared in the first worker 13 days after household visits began) and subsequently tested positive for SARS-CoV-2, resulting in testing of all other workers from May 16-19. A total of 12 (19%) tested positive: 8 were symptomatic and 4 were asymptomatic. After face shields were implemented, 50 previously uninfected workers visited 18,228 households with 118,428 people; 2,682 later tested positive for SARS-CoV-2. All workers were screened for symptoms and tested via RT-PCR for SARS-CoV2 weekly during this period. None of the 50 workers tested positive for SARS-CoV-2 infection from May 20 to June 30, 2020.
Workers had few sources of exposure to SARS-CoV-2 other than household visits to asymptomatic contacts of index cases. The protocols designed to limit transmission during the two periods appeared to be identical other than the exposure of interest. A large number of households were visited by workers, with many subsequent positive tests among household contacts, representing a considerable risk of SARS-CoV-2 transmission to workers during the face shield period.
The before-after comparison makes it difficult to be certain that other conditions did not contribute to the reduction in transmission to health care workers. For example, the positive test results in the pre-shield period may have encouraged behaviors and precautions among workers that were not measured. Testing procedures and indications were not well described, particularly during the post-shield period. This study assessed SARS-CoV-2 infection status using PCR (i.e., antigen testing) only; thus, some infected workers may have been missed if they were not tested while shedding detectable virus. Antibody testing would have improved interpretability of results.
This is the first study to date assessing the independent effectiveness of face shields in reducing the transmission of SARS-CoV-2.
This review was posted on: 25 August 2020