A new working paper from researchers at the University of California at San Francisco, “Comparison of infection control strategies to reduce COVID-19 outbreaks in the United States: A simulation study,” finds that in high-risk homeless shelter environments, a combination of intensive infection control strategies is unlikely to prevent COVID-19 outbreaks. The authors evaluate daily symptom screening, twice-weekly testing of all residents or just staff, universal mask wearing, and relocation of high-risk individuals. Using all these measures gives shelters only an 8% chance of averting an outbreak when the virus is highly infectious and spreads quickly. The authors argue that these findings show the need for continued non-congregate housing arrangements for people experiencing homelessness.
The authors measure the effectiveness of infection control strategies to slow the spread of COVID-19 in congregate shelter settings, using an epidemiological model that simulates transmission of SARS-CoV-2. The model was calibrated using the results of COVID-19 testing during outbreaks in five shelters in San Francisco, Boston, and Seattle in March and April. The model simulates the transmission of SARS-CoV-2 within a shelter of 250 residents and 50 staff (based on average shelter size) over 30 days, starting with one latently infected individual. Six infection control strategies were tested:
- Daily symptom-based screening of all individuals in the shelter
- Twice-weekly testing of all residents and staff
- Universal mask wearing
- Relocation of “high-risk” individuals (those 60 years or older or with co-morbidities) to single hotel rooms
- Twice-weekly testing of staff only
A combination of the first four strategies. For each intervention strategy, the microsimulation model was run 1,000 times, using different levels of infectiousness estimated from the data collected in San Francisco, Boston, and Seattle.
For each intervention strategy, the authors estimated the probability of avoiding an outbreak (defined as three or more infections originating within the shelter within a 14-day period). Daily symptom screening on its own performed poorly across all scenarios, as the chance of avoiding an outbreak ranged from just 3% to 33%, depending on the infectiousness of the virus. Relocating high-risk individuals and performing daily symptom screening performed just as poorly. Twice-weekly testing of staff raised the chance of avoiding an outbreak to 40%, but only when infectiousness was relatively low. Universal masking and twice-weekly testing of all individuals yielded better chances of avoiding an outbreak (between 6% and 47% and between 4% and 52%, respectively, depending on infectiousness). A combination strategy that involved daily symptom screening, twice-weekly testing of all individuals, universal masking, and removal of high-risk individuals gave the shelter the highest chance of avoiding an outbreak (8%-68%) but still only prevented a minority of outbreaks in most of the simulations.
The findings demonstrate the need for combined interventions and regular testing to protect people experiencing homelessness from COVID-19, but they also show the limitations of these interventions in high-risk settings. The authors argue that in the context of a highly infectious virus, non-congregate housing arrangements and social distancing are needed to limit the incidence of infection and severe disease.
The paper can be found at: https://bit.ly/36K5A1L