RSV, or respiratory syncytial virus, is a common virus that causes cold symptoms. In particularly, young children and vulnerable older people are more likely to become seriously ill if they get it. After the coronavirus measures were implemented in February 2020, RSV stopped circulating almost entirely, and the usual winter peak in RSV infections did not happen in the winter of 2020/2021. When most of the measures stopped in June 2021, RSV infections and resulting hospital admissions rose sharply. This resulted in an unusual peak in the summer of 2021.

RIVM research shows that this unusual summer peak may have been caused by lower levels of RSV-specific antibodies among people in the Netherlands. It appears that adults were the first to contract RSV infections again after most of the coronavirus measures were discontinued.

Research in France and the UK also shows that more people became ill due to RSV. This may have been because people were less likely to encounter the virus as a result of the coronavirus measures. But how did immunity to RSV decline due to the coronavirus measures? And could that explain the summer peak in RSV infections? These questions were investigated based on blood samples from the PIENTER Corona Study. 

PIENTER Corona Study measures immunity to SARS-CoV-2

Since 2020, the PIENTER Corona Study has been researching immunity to the coronavirus SARS-CoV-2 in the Netherlands. To that end, the RIVM study measures antibodies in the Dutch population. The blood samples from 558 participants in this study were also checked for RSV-specific antibodies. These randomly selected participants range in age from 1 to 89 years old.

Comparing blood samples

The researchers looked at the presence of IgG antibodies against the respiratory syncytial virus (RSV) fusion protein. These antibodies help to neutralise RSV. Researchers compared blood samples from 558 participants, taken in three different time periods:
-    June 2020 – a few months after the coronavirus measures were implemented.
-    February 2021 – about one year after the last normal RSV season.
-    June 2021 – several weeks after the coronavirus measures were discontinued and hospital admissions with RSV patients began to increase.
Comparing these blood samples showed the changes in immunity to RSV during the coronavirus pandemic.

Antibodies decreased overall, but increased in a small group

In the interval between the first and third sampling timepoint, IgG antibodies against the RSV fusion protein decreased significantly in nearly all age groups. The exception was people aged 31–40 years; that age group did not show a significant decrease. People aged 71 years and older had the highest levels of IgG antibodies. However, IgG antibodies increased in 9 of these 558 participants (1.6%), probably indicative of exposure to the virus. All of these people were over 30.

Increase in RSV started among adults?

The increase in antibodies among the small group of participants over 30 is striking. This could mean that adults were the first to contract RSV infections again, when the coronavirus measures were discontinued in June 2021. Adults generally have RSV infections more often than young children. They do not usually develop serious symptoms or end up in hospital. RIVM researchers found indications that the virus started circulating in the Netherlands more again after the coronavirus measures were discontinued. Decreased immunity to RSV may have led to the striking summer peak.

Substantiating findings with more timepoints and new blood samples

It is clear that monitoring antibodies has helped us understand why we saw an unusual summer peak in 2021, and who was infected first as the virus emerged. The study will continue with the aim of adding more sampling timepoints and new blood samples to substantiate these preliminary findings. The researchers will be specifically looking at which people were the first to get an RSV infection.