Peak at the end of 2023

In the summer of 2023, during the respiratory year 2022/2023, there was little circulation of the SARS-CoV-2 virus in the Netherlands. From July 2023 (weeks 26-29), virus circulation gradually increased.

In the respiratory season 2023/2024, wastewater surveillance indicated that after the gradual increase since July 2023 (from week 26), virus circulation increased stronger since the end of October 2023 (from week 43). The amount of SARS-CoV-2 virus particles in wastewater peaked in weeks 50-51 (December 2023). The percentage of Infectieradar participants reporting a positive self-test showed a similar pattern, with a gradual increase from week 29 and a peak in week 50 2023. The number of COVID-19-related hospital admissions, after a gradual increase since week 29, stabilized around an average of 100 admissions per day from week 43 to 48 in 2023 and then began to rise again to a peak of an average of 165 admissions per day in week 51. By the end of January 2024, the circulation of SARS-CoV-2 had returned to the level before the surge in November/December (weeks 46-52, 2023).

Subvariants of the coronavirus SARS-CoV-2

In the national SARS-CoV-2 pathogen surveillance, it was observed that from June to October 2023, various omicron (sub)variants of SARS-CoV-2 circulated alongside each other. From week 37 in 2023, the SARS-CoV-2-subvariant JN.1() began to appear in the national strain surveillance and grew to become the driver of the epidemic surge in the last months of 2023. This subvariant was responsible for more than 90% of SARS-CoV-2 cases by early 2024. It was the first omicron subvariant since BA.5 (in the summer of 2022) to reach such a high share. JN.1 has a growth advantage over previously circulating variants and possesses some antigenic advantage that evades prior immunity, but it does not seem to cause more severe illness than earlier omicron variants.

The number of measured SARS-CoV-2 virus particles in wastewater (4,343 particles x100 billion per 100,000 inhabitants) was the highest reported so far in week 51 of 2023. The growth advantage of the omicron subvariant JN.1, combined with possibly higher shedding in feces, could explain this high peak. Similar to the wastewater, the observed peak in Infectieradar participants with a positive test was higher than the previous surge in February-March 2023, but it was not as high as the surge in March 2022, after the omicron variants became dominant. The peak in hospital admissions in week 51 of 2023 (165 admissions) was comparable to the peak of 173 admissions in week 10 of 2023 during the previous surge in February/March 2023. SARS-CoV-2 does not yet have a stable seasonal pattern, and surges can also occur outside the respiratory season.

COVID-19 vaccination

The COVID-19 vaccination programme for people aged 60 and over and specific risk groups started in October 2023 (week 40). The timing of the vaccination programme may have contributed to the stabilization of the number of hospital admissions in October and November 2023, despite the increased virus circulation.