SARS-CoV-2 Antibodies Among Frontline Hospital Staff, Spring 2020

This page summarizes a study from early in the COVID-19 pandemic, with blood samples collected from April to June 2020.

Health care workers caring for patients with COVID-19 may be at high risk of catching SARS-CoV-2, the virus that causes it, and once infected they can pass it to patients, coworkers and the community. In spring 2020 little was known about how many frontline U.S. health care personnel had been infected. A CDC study with the Influenza Vaccine Effectiveness in the Critically Ill (IVY) Network set out to measure it by testing staff for antibodies, which most infected people develop within 1 to 2 weeks.

The study

Thirteen academic medical centers in 12 states took part. From April 3 to June 19, 2020, each enrolled volunteers who had regularly worked directly with adult COVID-19 patients since February 1, 2020, in emergency departments, intensive care units and hospital wards, aiming for 250 per hospital. Staff who were off work because of illness or quarantine were not enrolled.

Participants gave a blood sample and answered questions about their background, health, any symptoms since February 1, earlier COVID-19 testing, their use of personal protective equipment (PPE) in the past week, and whether they had faced a PPE shortage. CDC tested the samples with an assay that detects any type of antibody against the virus's spike protein, with a sensitivity of about 96% and a specificity of 99% in earlier validation.

Of the 3,248 participants, 44% were nurses; 28% were physicians, nurse practitioners or physician assistants; 7% were respiratory therapists; and 20% had other clinical roles. Their median age was 36, and 80% reported no underlying medical conditions. Forty percent worked mainly in an intensive care unit, 35% mainly in an emergency department and 25% elsewhere.

What the study found

• 6.0% had antibodies. 194 of 3,248 participants tested positive. The rate varied widely by hospital, from 0.8% (at three facilities) to 31.2%, with a median of 3.6%, and was generally higher where the surrounding county had more COVID-19 cases.
• Many infections went unnoticed. Of the 194 with antibodies, 56 (29%) reported no symptoms of a viral illness since February 1, 86 (44%) did not think they had had COVID-19, and 133 (69%) had never had a positive test for active infection.
• Symptoms were more common among those infected: 71% of those with antibodies reported symptoms of an acute viral illness, against 43% of those without.
• Differences by group. Antibodies were less common among women (5.3%) than men (7.2%), and among non-Hispanic White participants (4.4%) than among participants of other racial and ethnic groups (9.7%).

Masks and PPE shortages

In the week before enrollment, 2,904 participants (89%) said they wore a face covering — defined as a surgical mask, N95 respirator or powered air-purifying respirator — during every clinical encounter. Antibodies were found in 5.6% of them, against 9.0% of those who did not always wear one.

A shortage of some form of PPE since February 1 was reported by 398 participants (12%), most often of N95 respirators; at eight of the 13 hospitals, more than 10% of participants reported one. Antibodies were more common among those who had faced a shortage (9%) than among those who had not (6%).

What it means

The authors conclude that some infections among frontline staff went undetected and unrecognized, perhaps because they caused few or no symptoms, because symptoms were underreported, or because testing of staff with symptoms was not systematic. They point to maintaining PPE supplies, masking for every interaction between staff and patients, frequent testing of frontline staff and lower thresholds for testing as ways to reduce spread in hospitals, and note that universal masking has been associated with a significantly lower rate of infection among health care workers.

Limitations

• Volunteers may have differed in risk from those who did not volunteer, and staff off sick or in quarantine — possibly at higher risk — were not included.
• Infections could be missed if antibodies had not yet formed or had declined.
• Hospital infection-control practices were not recorded, and the analysis did not adjust for confounding.
• The study could not tell whether infections were acquired in the hospital, including from coworkers in shared workspaces, or in the community.

Sources

• Wesley H. Self, Mark W. Tenforde, William B. Stubblefield, Leora R. Feldstein and colleagues, CDC COVID-19 Response Team and IVY Network, "Seroprevalence of SARS-CoV-2 Among Frontline Health Care Personnel in a Multistate Hospital Network — 13 Academic Medical Centers, April–June 2020," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/69/wr/mm6935e2.htm
• The report's figure is not reproduced.