  Due to its high lethality amongst the elderly, the safety of nursing homes
has been of central importance during the COVID-19 pandemic. With test
procedures becoming available at scale, such as antigen or RT-LAMP tests, and
increasing availability of vaccinations, nursing homes might be able to safely
relax prohibitory measures while controlling the spread of infections (meaning
an average of one or less secondary infections per index case). Here, we
develop a detailed agent-based epidemiological model for the spread of
SARS-CoV-2 in nursing homes to identify optimal prevention strategies. The
model is microscopically calibrated to high-resolution data from nursing homes
in Austria, including detailed social contact networks and information on past
outbreaks. We find that the effectiveness of mitigation testing depends
critically on the timespan between test and test result, the detection
threshold of the viral load for the test to give a positive result, and the
screening frequencies of residents and employees. Under realistic conditions
and in absence of an effective vaccine, we find that preventive screening of
employees only might be sufficient to control outbreaks in nursing homes,
provided that turnover times and detection thresholds of the tests are low
enough. If vaccines that are moderately effective against infection and
transmission are available, control is achieved if 80% or more of the
inhabitants are vaccinated, even if no preventive testing is in place and
residents are allowed to have visitors. Since these results strongly depend on
vaccine efficacy against infection, retention of testing infrastructures,
regular voluntary screening and sequencing of virus genomes is advised to
enable early identification of new variants of concern.
