The Spanish results are based on tests of more than 60,000 people across the country. The study was carried out by the Carlos III Institute and the Spanish equivalent of Statistics Sweden.

Preliminary results show that about five percent of the population carry antibodies and thus have been infected with covid-19.

Can be twice as many

But in fact, 5 percent may already be 10 percent. And even 20 percent or more in metropolitan regions. This is what Anders Björkman, professor of infectious medicine at the Karolinska Institute, believes.

- It was a fairly expected result that has probably doubled today, he says.

It takes about two weeks for the body to develop antibodies from the time you contract covid-19, he explains.

The tests in Spain were conducted between April 27 and May 11. A median date will then be May 4, which means that many of the people tested became sick by about May 20, several of them probably earlier than that.

Anders Björkman therefore believes that the results presented now reflect the proportion of infected about a month ago. He believes that the amount of infected has doubled since then.

Corresponds to Stockholm

The Spanish study also shows major regional differences in the country. In the Madrid area, 11.3 percent of the population had antibodies. If you count on a doubling, today's figures fall to just over 22 percent.

- This is a sum that is quite in line with Stockholm, where we believe that just over 20 percent have been infected. It might even start approaching 30, says Anders Björkman.

In addition, the results show that the antibody tests do not catch all infected.

In the study, the question was asked whether the tested subjects had previously been diagnosed with covid-19. Among the group that responded that they were diagnosed, 87 percent were on antibodies.

This means that 13 percent of those diagnosed with covid-19 showed negative results on the antibody test, which in turn indicates that the real number of infected may be even higher, says Anders Björkman.

Hard to compare

In Sweden, several smaller studies indicate that more have been infected than the study in Spain shows. State epidemiologist Anders Tegnell tells TT that comparisons of the proportion of infected are difficult to make.

- I would probably argue that for this disease we will probably never be able to transfer data of this type from country to country, he says.