Study: SARS-CoV-2 attack rate was low in schools — even lower with masks, vaccines

Key takeaways:

• Masking and vaccination were the strongest factors associated with fewer COVID-19 cases in schools.
• Protective measures are still needed even at this point in the pandemic, a researcher said.

A study of SARS-CoV-2 transmission in 10 Massachusetts school districts found that the secondary attack rate of SARS-CoV-2 among school contacts was low — between 2% and 3% — and that masking reduced the odds of transmission by almost 90%.

Being fully vaccinated was associated with even lower odds of transmission, whereas being exposed in a classroom vs. outside the classroom more than doubled the odds.

The study assessed transmission in the school districts during the 2020-2021 school year and the fall of 2021. COVID-19 case counts are lower now, but the authors of the study said COVID-19 surveillance efforts in schools may still be “essential.”

Aligns with past results

Prior research has also shown that masking is associated with fewer COVID-19 cases in schools. Studies in the post-pandemic period have also found that COVID-19 cases spiked in schools that lifted mask mandates.

“When we started this study, there was a real void of information about factors associated with COVID-19 transmission in schools,” Sandra B. Nelson, MD, a physician at Boston General Hospital, told Healio.

“After the initial school closures in the spring of 2020, schools were opening for in-person learning. We had an understanding that certain interventions — such as masking, distancing, and screening testing programs — could reduce the potential for transmission, but we didn’t know how these measures would perform in school settings,” Nelson said. “This study was designed to answer those questions.”

Low secondary attack rate

Students and staff became eligible for COVID-19 vaccination during the study periods. During the 2020-2021 school year, the researchers studied data from eight districts with 70 schools and over 33,000 students. Data from the fall of 2021 include five districts with 34 schools and over 18,000 students.

“This study built upon contact tracing that schools were required to perform during the early pandemic,” Nelson said. “School health personnel were tasked with identifying students and staff who were exposed to COVID-19 so that they could be quarantined and tested.

“We developed an enhanced contact tracing tool for school health personnel to use to track these exposed persons, while also gathering additional information about the nature of the exposure itself, including the location of exposure, the use of masks, and the vaccination status of case and contact, and whether or not the contact tested positive.”

These measures allowed Nelson and colleagues to determine a secondary attack rate in the schools — the likelihood an exposed person would ultimately test positive for COVID-19 — and whether any features of exposure are associated with transmission.

In the 2020-2021 period, schools reported 435 index cases of SARS-CoV-2 infection with 1,771 school-based contacts. In the fall of 2021, schools reported 309 index cases with 1,673 school-based contacts.

The secondary attack rate — or the proportion of school-based contacts who acquired SARS-CoV-2 from a case — was 2.2% (lower bound, 1.6%; upper bound, 26.7%) during the 2020-2021 school year and 2.8% (lower bound, 2.6%; upper bound, 7.4%) during the fall of 2021.

The researchers found that the factors associated with transmission risk varied over time

“First, we were relieved — but in truth, not surprised — to learn that the likelihood of transmission in school settings was low during the time periods we studied,” Nelson said.

“Most persons, about 97% of those tested [who were] exposed to SARS-CoV-2 in schools, did not test positive for the virus. Further, we were not surprised to learn that both masking and vaccination were protective against transmission during different phases of the pandemic. However, we were surprised to learn that even when these factors were taken into consideration, individuals exposed to SARS-CoV-2 at schools in more socially vulnerable communities were more likely to test positive for COVID-19 after exposure.”

Nelson and colleagues argued that ongoing surveillance may be essential to ensure that appropriate resources and mitigation strategies remain in place.

“We are clearly in a different place in the pandemic in terms of both the severity of illness we are seeing and our comfort with the virus in our midst,” Nelson said. “But this virus and likely others in the future will continue to threaten in-person learning, ... While many studies have demonstrated that vaccination reduces illness burden, this study demonstrated that vaccination can also be thought of as a tool to prevent learning loss by keeping our students and staff in school.”