Residential care centers for elderly were particularly hit by the first COVID-19 wave. Two-thirds of all COVID-19 deaths in Belgium can be attributed to that group and that period. Residents of residential care centers are typically older and generally have weaker health. At the same time, they often have fewer social contacts and mobility. This makes this group a "reservoir" for infections: despite a high morbidity and mortality (in other words, a high proportion of the sick and a high proportion of deaths), this group does represent a driving force for virus spread in the broader society.

To better understand how a virus like SARS-CoV-2 is transmitted between the broader community and this particularly vulnerable group of people, colleague Nicolas Franco worked on an extension of our SEIR model that includes residential care centers as separate communities. We modeled infections between 'compartments' of susceptible, exposed, infected, recovered, and deceased COVID-19 patients for 2,000 residential care centers, in addition to the spread in the general population. We also took into account the fact that testing was not always carried out in the first wave, and therefore not all deaths in residential care centers were necessarily classified as COVID-19 deaths. The modeling of the number of hospitalizations became more complex because whether or not residents from residential care centers were hospitalized depended on the occupancy rate of the respective hospital.

We also refined the part of the model that maps the spread in the general community by taking into account the reintroduction of COVID-19 infections by travelers, and for residential care centers, the introduction via visitors.

We can use this model to assess the effect of various scenarios, for example, in the context of new measures. Through this modeling approach, we calculated that the effective reproduction number (Rt) could be significantly reduced in March and April 2020 to as low as 0.6. Rt becomes greater than one when the second wave begins in September. The infection fatality rate hovers around 1% for the general population, with a clear age-related relationship: 0.01% (or 1 in 10,000) for children and young people, up to more than 8% for those aged 75 and older.

However, like any model, this version is an approximation of reality. For example, the model does not account for the geographical distribution of the 2,000 residential care centers in the country. In reality, some municipalities have multiple residential care centers, while others have none, which is a significant factor in transmission, even in a small and densely populated country like ours.

While we conducted targeted research on how infections spread in schools or between family members long before the pandemic, there is actually very little data on the specific circumstances in residential care centers, where ultimately the most vulnerable people live. During the first wave of the COVID-19 crisis, it became painfully clear that more attention is needed for this group, not only in terms of care but also in terms of research.