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THL: Antibody testing suggests severe second wave of infections likely

The public health agency THL found Covid-19 antibodies in just 2.4 percent of samples tested.

Akatemiatutkija Tomas Strandin laboratoriossa 14.4.2020
Researcher Tomas Strandin examines coronavirus antibodies at a lab in Meilahti, Helsinki. Image: Kalevi Rytkölä / Yle

Novel coronarivus has not spread as widely in Finland as previously thought, according to new antibody test results. The data suggest that models previously prepared for the government are based on the assumption that more people have been exposed to the disease and have acquired immunity to it than have actually done so.

According to the results of research released by the Finnish Institute for Health and Welfare (THL) on Friday, antibodies to the virus were found in just 2.4 percent of 1,146 test samples. Meanwhile neutralising antibodies -- the surest indication of a Covid-19 infection -- were found in just three samples.

The test samples were collected between 20 April and 3 May from persons between the ages of 18 and 69. However the test results reflect a slightly earlier phase of the progress of the disease given that antibodies develop at least two weeks after infection.

The majority of samples came from the Helsinki and Uusimaa hospital district (HUS). Because the prevalence of infections in that region is higher than in the rest of the country, the results do not provide an accurate picture of the spread of the disease throughout the entire country. The presence of antibodies in other parts of the country is therefore believed to be even lower.

"It looks like we have very few infections compared to many other countries. In addition, increased infections do not appear to have occurred in the weeks after the samples [were taken]," THL specialist researcher Merit Melin said.

Actual situation unclear in March

The antibody test results appear to align with data released one week ago and bolster the view that previous models have exaggerated estimates of coronavirus infections.

For example, a report published on Monday by Finance Ministry permanent secretary Martti Hetemäki and used by the government to arrive at a decision on gradually re-opening society, is based on the assumption that by mid-March, 2.5 percent of people in Finland would have been infected and would have developed neutralising antibodies. However the new data suggests that this assumption was too optimistic.

THL said that that assumption was based partly on an initial analysis of surplus blood samples from a HUS lab. Surplus serums do not reflect the population as well as random samples pulled from the population.

"It was the best estimate at the time," said Turku University statistics professor Kari Auranen, a member of THL's modelling team.

Scenarios dependent on human behaviour

From the Finnish perspective it would be better if more people had contracted Covid-19 by now. This would mean that a possible second wave of the epidemic would not be as severe. This view is based on the assumption that people who do get infected will gain some form of immunity from the virus.

When THL published the background to its models, it also laid out dozens of different scenarios to describe the severity of the next wave of infections. These scenarios vary depending on how much we limit interpersonal contact and how many of us have already been infected.

According to the forecasts in the Hetemäki report, Finland's intensive care unit capacity would just about hold up if restrictions were eased for four months so that the R0 value -- the number that describes the disease's level of contagion -- would rise to 1.8, meaning that on average one infected person would pass on the virus to 1.8 others.

But since the latest data suggest that far fewer have developed immunity to the disease, the next wave of infections could hit harder than anticipated. This would put even more pressure on hospitals than forecasted in Hetemäki's report.

However Auranen pointed out that no scenario plays out exactly as plotted. Models have been updated throughout the spring and will continue to be revised as new information comes to hand.

"We have not committed to anything. The future will probably be such that we will monitor the situation constantly and policies will have to change," he commented.

By Friday, Finland had reported 5,738 lab-confirmed cases of Covid-19, the disease caused by novel coronavirus.

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