What trends are observed?|
Disappearance and return of respiratory viruses
Combined data1 from KK Women’s and Children’s Hospital (KKH), Singapore General Hospital and National University Hospital showed that after a peak of respiratory virus activity around December 2019 and January 2020, and subsequent implementation of COVID-19 control measures, most non-COVID-19 respiratory viruses had nearly disappeared by April-May 2020 (Circuit Breaker period).
With gradual easing of COVID-19 measures in Singapore, the first respiratory viruses to return, in late 2020, were the rhinoviruses and enteroviruses. These are viruses that usually cause mild common colds, although enteroviruses can also cause hand-foot-mouth disease (HFMD) and meningitis. The next respiratory viruses to return, in early 2021, were respiratory syncytial virus (RSV) and some of the parainfluenza viruses. These are viruses that may cause more serious illness in young children, such as bronchiolitis from RSV or croup from parainfluenza viruses.
Increased RSV activity
In other countries, Australia reported2 increased RSV activity even in late 2020, which is outside the usual RSV season associated with winter months in the northern or southern hemisphere. In 2021, Europe reported3,4 waves of RSV infections, similarly outside the traditional winter months. RSV infections were also seen in children who were older than usual, extending to those above one year old. At times, the number of RSV infections even surpassed that of COVID infections.
What had happened was that young children born during 2020 have had much less exposure to common respiratory viruses than same-age children in previous years. This had created an ‘immunity debt’5 towards previously common infections, meaning that the lack of exposure to viruses and bacteria during the pandemic left people more susceptible to them, and this is particularly relevant for young children.
Classical influenza viruses
Many different viruses can cause respiratory infections, but in a strict medical sense, only influenza viruses cause classical influenza or ‘the flu’. Interestingly, the classical influenza viruses – influenza A and influenza B viruses – also nearly disappeared during the pandemic. Influenza viruses also remained near-absent in Singapore during the second half of 2020 and throughout 2021. A small number of influenza infections were observed again in the first half of 2022, according to data from KKH. These numbers are expected to rise in the second half of 2022, due to the opening up of travel internationally. Influenza infections may be more serious in young children, pregnant women, and the elderly.
Pandemic response measures have not only affected respiratory viruses, but also bacteria. Before the pandemic, KKH regularly diagnosed infections with Mycoplasma pneumoniae6 (which causes pneumonia predominantly in children) and Bordetella pertussis (the agent of classical whooping cough). These infections similarly disappeared during the 2020 Circuit Breaker and did not reappear until the time of writing, mid-2022.
Is there cause for concern?|
Respiratory viruses have been with us for a very long time. They have shaped our population and their collective immune system to what it is today. In essence, they are part of what helps train our immune systems and those of our children. Their absence, rather than presence, is an unusual state. Their resurgence after the initial phases of the COVID-19 pandemic should be seen as part of the return to normality, but may also be cause for concern, due to the possibility of severe illnesses in some children and vulnerable adults. Thus, the return of many respiratory viruses is not unexpected and means that many of us will again have flu-like illnesses off and on.