For SARS-CoV-2, the Nordic regions had a relatively low number of cases during the pandemic and the following years compared to other European countries, however there was great disparity between the countries in timings and excess mortality.4 Sweden experienced high excess mortality in 2020, and lower in 2021, whereas Denmark, Finland and Norway experienced the opposite, with excess mortality peaking in 2022.4 This may partly be explained by the early roll out of the COVID-19 vaccine in Sweden.4

The demand on healthcare services each winter is further complicated by rates of RSV increasing across most of Europe, including the Nordic region.5 It is estimated that 28 to 60 percent of respiratory tract infection hospitalisations of children under three months are associated with RSV in Denmark, Norway and Sweden,6 and all three countries have reported three or more consecutive weeks of RSV positivity above three percent since 2022.5

Emerging viral threats

The predominant strains of each virus vary year-to-year, but they can also fluctuate within a single season; the first peak in the 2022/23 influenza season in Norway was dominated by A(H1N1) strains, while the second featured significant levels of both A(H3N2) and B/Victoria viruses, and the final was dominated by B/Victoria-lineage viruses.1 Understanding the prevalence of influenza strains is critical as, occasionally, strains appear that create a major public health emergency. For example, particularly virulent strains of influenza A(H1N1) were responsible for the 1918 and 2009 pandemics, with the former resulting in an estimated 50 million deaths worldwide.7 The constant evolution of new influenza strains in birds has forced the European Centre for Disease Prevention and Control to encourage enhanced influenza surveillance during the coming 2024 winter season, with guidance on influenza A and B testing for all patients admitted due to respiratory or other influenza related symptoms.8 As a result, the Public Health Agency of Sweden has tightened surveillance of influenza.9

The hidden threat of new strains

PCR testing offers greater specificity and sensitivity compared to immunoassay or culture-based methods,10 positioning the technology as the most effective method for monitoring respiratory viruses. However, the rapid evolution of novel viral strains, driven by their ability to cross between a wide range of hosts,8 also threatens the effectiveness of even these molecular tests, as these viruses can mutate to evade detection. A 2022 study found notable issues in the detection of A(H3N2) during the 2021-2022 influenza season in Denmark.10 Mutations in the primer binding sites of the matrix gene in A(H3N2) enabled it to evade detection by several commercially available diagnostic platforms, resulting in high rates of false negatives, particularly from assays that rely solely on the matrix gene for detection.11