Covid-19 prevalence over two times higher than confirmed cases, study reveals

870,000 Catalans have developed Covid-19 antibodies according to a study carried out by Spain's health ministry and the Carlos III Institute of Health in Madrid.

Results, which were made public on Tuesday following a fourth round of tests, indicate that 11.6% of Catalonia's residents – that is to say, over twice the official number of people who have been diagnosed since the beginning of the pandemic – have actually had the virus: 12.4% in Barcelona; 12.2% in Lleida; 11.4% in Girona, and 5.6% in Tarragona.

Meanwhile, it appears that almost 1 in 10 Spaniards, or 9.9% of the population, has been exposed to the virus.

How the study works

A portion of the population randomly selected by Spain's statistics institute, the INE, is asked to undergo a rapid serological test.

Some 10 minutes after blood is taken from a pricked finger, the result shows if the subject has developed antibodies against the virus, with blood also taken for a lab test and a survey done to obtain more information, such as possible symptoms.

So far, there have been four rounds of tests, with 51,409 people from Spain's 17 regions and 2 autonomous cities participating in the fourth round last November. A fifth round of tests will be carried out in February and a sixth will take place in May.

What rapid tests reveal

Participants are given the results of the rapid test, which provides information on two types of defenses, Immunoglobulin M and G (IgM and IgG). 

If the result is positive in IgM, it means that person is at an early stage of infection, as these are the first antibodies to be generated.

"If the test shows elevated IgGs, which are the defenses that appear later, it means that person has been through the infection and developed protection," says the head of care of Barcelona's CAPs, Dr. Belén Enfedaque.

"If both come out, it means the person is in a phase where they are healing and late defenses are appearing. In this case, a visit should also be made to find out what point the infection is at, whether you have had symptoms that may have gone unnoticed, and the necessary action to be taken," she adds.

Far from herd immunity

Not everybody who has developed Covid-19 antibodies still has them.

Indeed, the overall 11.6% figure for Catalonia and 9.9% figure for Spain is the percentage of people that have had antibodies since the first round of tests was carried out.

When considering results exclusively from the fourth round, these numbers are lower: 9.2% for Catalonia as a whole, with 9.8% in Barcelona, 10.4% in Lleida, 8.7% in Girona, and 4.4% in Tarragona. For Spain, the figure stands at 7.2%.

Marina Pollán, the head of epidemiology at Carlos III, was cautious about drawing conclusions from this, explaining that it was not yet clear whether this indicates a loss of immunity.

She does, however, believe the population as a whole is far from becoming immune to the virus as still not enough people have overcome the virus.

"We will have a vaccine before we achieve herd immunity," she said. "Whatever the required immunity percentage may be – we are still discussing that – it is much higher than 10%."