Early estimates of the spread of SARS-CoV-2 continue to hold up

Does anyone remember the findings published by the Finnish Institute for Health and Welfare in spring 2020 on the spread of the coronavirus epidemic?

Estimates of the severity of the COVID-19 disease and the actual number of coronavirus infections were presented early in the pandemic. While the first results were obtained very quickly, an analysis by Doctoral Researcher Tuomo Nieminen at the Department of Mathematics and Statistics, University of Helsinki, recently published in the refereed Plos One Journal, confirms the underreporting of SARS-CoV-2 infections.

While experts from the Finnish Institute for Health and Welfare appeared daily in news and current affairs programmes reporting on the disease situation, Nieminen and other researchers behind the scenes at the institute produced the percentages and predictions presented by feeding datasets into mathematical models.

The laboratory-confirmed infections were recorded as COVID-19 disease cases in the Finnish National Infectious Diseases Register in almost real time. However, the virus testing only targeted the symptomatic cases, and hence not all infections could end up in the register. There was a demand for a quick understanding of the number of asymptomatic cases, and whether a part of the population had already developed immunity to the virus. However, this was no easy feat.

A serological population study was quickly launched, with a representative sample of citizens invited to participate. Blood samples were used to determine the amount of antibodies associated with coronaviruses.

The recently published article confirms the validity of the estimates on the number of undetected infections produced in the first quickly conducted analyses during spring 2020.

“It was a relief that no unexpected weaknesses emerged while writing the research article. Now, the rapidly developed innovations and work carried out under pressure have been validated and documented,” says Tuomo Nieminen.

Comparing antibodies and disease cases is a well-known method. In this study, an added complication was the fact that the novel coronavirus and its characteristics were not well known at the time. In their analysis, the researchers at the Finnish Institute for Health and Welfare took into consideration the data from a study conducted in China in early 2020 on how rapidly antibodies to SARS-CoV-2 develop in different individuals, as there is individual variation.

According to the recently published study, the actual number of SARS-CoV-2 infections in Finland before April 2020 was 4 to 17 times the number of registered COVID-19 cases. However, the situation quickly changed as virus testing capacity increased, thanks to which a larger share of infections could be detected. In the spring of 2020 overall, the number of infections "was equal to or up to five times higher than that of detected disease cases. During the period, approximately 7,200 cases of COVID-19 were recorded in Finland. 

“The research method compared the proportion of registered COVID-19 cases with seroprevalence; the proportion of individuals with antibodies to SARS-CoV-2, which was assessed with the help of random population sampling and blood samples,” Nieminen says. “A delay in the development of antibodies must be taken into consideration in the comparison. By using a statistical model, we predicted the probability of antibody development for each individual case of COVID-19. Summing up these probabilities made direct comparison with seroprevalence possible.”

“The analysis provided an estimate of underreporting; the proportion of detected infections from all infections. Thanks to methodological innovations, it was possible to estimate the underreporting and its trend over time more accurately compared to previous corresponding studies,” Nieminen says.

Even though a significant share of infections most likely also went undetected in Finland in the spring of 2020, the number of infections in the country was still considerably lower compared to other European countries. It is likely that less than 1.5% of the population had a coronavirus infection in spring 2020 in Finland.

From a career in poker to statistics

Following roughly 10 years as a professional poker player, Tuomo Nieminen began studying statistics in 2012. His career at the Finnish Institute for Health and Welfare began through a summer traineeship, which led to him finding the topics for his thesis. Nieminen’s first supervisor, a close colleague even today, was Jukka Jokinen, currently serving as a professor of practice at the University of Helsinki.

“Working at the Finnish Institute for Health and Welfare to promote public health is motivating and provides an opportunity for societally beneficial work,” Nieminen notes.

A doctoral thesis project initiated in 2019 alongside his work, put on the backburner in the height of the coronavirus years, is now regaining speed.

Nieminen’s primary interest lies in investigating the adverse effects of vaccinations.

“All drugs as well as vaccines have adverse effects, and it’s really important that research is conducted and that it is transparent. Citizens’ trust in vaccines, their safety and vaccination programmes can only be achieved by actively investigating safety,” Nieminen says.

Further information

Nieminen TA, Auranen K, Kulathinal S, Härkänen T, Melin M, et al. (2023) Underreporting of SARS-CoV-2 infections during the first wave of the 2020 COVID-19 epidemic in Finland—Bayesian inference based on a series of serological surveys. PLOS ONE 18(6): e0282094. https://doi.org/10.1371/journal.pone.0282094