The national study has looked at all cases of established covid-19 up to and including September last year.

This is a total of more than 84,000 people.

The researchers have used registers to compare these cases with a control group of over 430,000 randomly selected people from the rest of the population.

With the help of this, they have looked at the connection between 34 possible risk factors and covid diagnosis, with or without hospital care.

The study shows that old age is by far the strongest risk factor for becoming seriously ill.

Other strong risk factors are male gender and underlying disease.

It was also seen that people with Down syndrome are at particularly high risk and people with home care have an increased risk that can not only be explained by age.

- The results show, among other things, that people with Down syndrome had a four times higher risk of becoming seriously ill with covid-19 than others.

We were therefore very happy when the Public Health Agency after we sent them the study announced that people with Down syndrome should be given priority for vaccination, says Peter Nordström, professor of geriatrics at Umeå University.

"One of the largest studies in the world"

The study also shows that asthma and high blood pressure increased the risk of hospitalization for covid-19 by 20 to 50 percent.

As only diagnoses from specialist care were available, the results for asthma could only apply to severe asthma.

The researchers think it is important to weigh the results of several studies and not focus too much on a single study,

- But a significant strength of the study is that it covers the entire population.

This makes the study one of the largest studies in the world in terms of risk factors for severe covid-19, which is important for generalizability.

Not least the information about specific risk factors such as Down syndrome is important, says Marcel Ballin, doctoral student in geriatrics at Umeå University and co-author.

The study is published in the scientific journal European Journal of Epidemiology.