One of the most important questions with any infectious disease, but particularly novel ones like SARS-CoV-2, is the mechanism and efficiency by which it spreads. From the beginning of the pandemic, there have been varied estimates of SARS-CoV-2 transmission rates and debate about whether it spreads predominantly through droplets versus aerosols.
In our published in Clinical Infectious Diseases, we sought to provide new insight into these questions by examining the risk of transmission among hospital roommates when one of the two had an undiagnosed SARS-CoV-2 infection.
Hospital roommates can offer unique insights into the risks and mechanisms for transmission because hospitalized patients typically spend most of their time in bed or next to their beds, usually more than six feet apart, with a curtain between them, and with minimal direct interaction. In addition, it is often possible to quantify the infectiousness of the source patient by assessing their SARS-CoV-2 viral load and the amount of time that the two parties overlapped while the source patient was potentially contagious. These conditions can be very helpful for elucidating mechanisms and risk factors for transmission.
Our study was conducted at Brigham and Women's Hospital in Boston between September 2020 and April 2021, a period of high disease incidence in Massachusetts. By this time, our hospital had implemented multiple protocols to prevent nosocomial spread of SARS-CoV-2 including universal masking of hospital employees, routine screening of patients for possible COVID-19 symptoms, isolation of suspected cases, universal PCR testing of all patients on admission and 72 hours later (to identify infections that were incubating on admission), and liberal testing for any new symptoms concerning for COVID-19.
How Many Roommates Became Infected?
During the study period, over 25,000 patients were admitted to the hospital, 599 of whom tested positive for SARS-CoV-2 on admission. There were 11,290 patients admitted to shared rooms. Of these, 25 tested positive after admission to a shared room (the "index cases") and thus put their roommate at risk of infection. These patients had 38 roommates in total during the period they were potentially infectious and some of the infected patients had more than one roommate before they were diagnosed. Follow-up testing at least 3 days post-exposure was available in 31 of these patients. Of these 31 patients, 12 were infected, translating into a secondary attack rate of 39%.
What Were the Risk Factors?
We attempted to elucidate potential risk factors for infection by comparing the characteristics of index patients who transmitted infections and roommates who acquired infections to their non-transmitting and uninfected counterparts. We considered duration of exposure, viral load (through PCR cycle threshold values), whether the source patient was symptomatic, whether nebulizers were used, if there were any so-called "aerosol generating procedures," and whether the source patient was agitated or yelling. We also evaluated exposed roommates' demographics (age, race, sex), co-morbidities, and whether their bed was near the room's vent or on the opposite side of the room.
On multivariable analysis we found that the only factor significantly associated with transmission was the index positive roommate's viral load: PCR cycle thresholds of ≤21 were significantly associated with transmission. This echoes other contact tracing studies that have also found strong associations between viral loads and transmission risk. Other factors were suggestive, but the small number of cases in our analysis limited statistical power to draw firm conclusions. Importantly, very few of the study subjects had been vaccinated, but one fully vaccinated roommate was infected by an unvaccinated index case.
What Can We Learn From This?
There are several important takeaways that can help improve infection control practices in the hospital as well as inform public health efforts to prevent transmission in the community. We found that SARS-CoV-2 is very easily transmitted if sharing a room with a person who is highly infectious, even if they are asymptomatic at the time (and importantly, viral loads tend to be highest right before symptom onset). High transmission rates occurred despite the fact that hospital rooms tend to have better ventilation than most homes and workplaces (≥6 air changes per hour vs ≤2). The high transmission rate coupled with the unique circumstances of hospital roommates (typically >6 feet apart, limited face-to-face contact, curtain between beds) provides further evidence that SARS-CoV-2 can be transmitted by . Indeed, the advantage of studying transmission in hospital roommates is that it minimizes many of the confounding factors that could explain the high transmission rates in household members, who are more likely to have close contact including hugging, kissing, sharing meals, or sleeping in the same bed. While it is possible that transmission was via fomites on doorknobs or the toilet or sink, there has been little evidence of spread of SARS-CoV-2 via fomites in the literature.
Several strategies may reduce transmission risk between patients that share rooms. Options include only pairing fully vaccinated patients together, placing portable HEPA filters between patient beds, increasing the frequency of air changes in rooms; performing more frequent serial testing of patients in shared rooms, and instituting stricter masking policies for patients that share rooms.
It is important to recognize, however, that while the risk of transmission from an infected patient to a roommate is very high, the net risk of someone being admitted to a shared room with a patient with unrecognized SARS-CoV-2 is very low in hospitals with aggressive screening and testing protocols. In our study, this only occurred in about 3 in 1,000 admissions. Hospitals and patients therefore need to weigh the potential benefits of these preventive strategies against their feasibility and cost given the rarity of occult positive admissions to shared rooms in hospitals that have robust infection prevention measures at baseline.
Our study can also help to improve public health efforts in the community, where people are more likely to be early incubators who are highly infectious and more likely to be living or working in poorly ventilated workspaces. The rise of more transmissible variants, such as Delta, as well as plateauing vaccine rates in the U.S. make it an urgent task to further reduce transmission risk for those who are most vulnerable.
is an infectious disease fellow in the Division of Infectious Diseases and Geographic Medicine at Stanford University. is an assistant professor of Population Medicine at Harvard ľֱ School and an infectious disease physician at Brigham and Women's Hospital. is a professor of Population Medicine at Harvard ľֱ School and an infectious disease physician at Brigham and Women's Hospital.
Disclosures
Karan has received consulting fees from the World Health Organization's Independent Panel for Pandemic Preparedness and Response for unrelated work. Rhee and Klompas have received grant funding from CDC and AHRQ, and royalties from UpToDate.