​​What effects have COVID-19 restrictions had on respiratory viruses?

During the COVID-19 pandemic, many countries implemented pandemic response measures, such as lockdowns, travel restrictions, mandatory mask-wearing in public or in workplaces, limitations on gatherings, and social distancing. These ‘horizontal measures’ had effects not only on the coronavirus causing COVID-19, but also on a range of other respiratory viruses, such those involved in common colds, and influenza.

In Singapore, pandemic restrictions were first implemented in February 2020, followed by a lockdown in April 2020 (Circuit Breaker). This was followed by a gradual reopening in phases (Phase 1-3). These measures were accompanied by changes in the prevalence of non-COVID respiratory viruses. During implementation of pandemic response measures, the prevalence of these viruses went down, and after easing of restrictions, they went up again. Similar effects were seen in other countries.

​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.

Immunity debt
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.

Bacterial infections
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.

​What can be done to protect children?

Parents should take good care of oneself and the children, seek medical care when the infections look to become more serious, and keep influenza vaccinations up to date. The Singapore National Childhood Immunisation Schedule recommends influenza vaccination annually or per season for all children aged six months to under five years and children aged five to 17 years with specific medical condition or indication. Anyone above six months of age can obtain their influenza vaccination at their local community clinic.

​What can the healthcare professional do to help?

Healthcare professionals should be on the lookout for classical influenza, enterovirus and RSV infections, and the latter may have a more serious impact in young children. Symptoms in children which may prompt a referral for tertiary assessment include:
  • Worsening or persistent fever
  • Lethargy or drowsiness
  • Persistent cough
  • Breathlessness
  • Chest pain
  • Poor oral intake
  • Poor urine output
  • Seizures
Healthcare professionals may make a referral to KKH at 6692 2984 or centralappt@kkh.com.sg.

  1.  Wan WY, Thoon KC, Loo LH, Chan KS, Oon LLE, Ramasamy A, Maiwald M. Trends in Respiratory Virus Infections During the COVID-19 Pandemic in Singapore, 2020. JAMA Netw Open. 2021 Jun 1;4(6):e2115973. https://doi.org/10.1001/jamanetworkopen.2021.15973.
  2. Eden JS, Sikazwe C, Xie R, et al. Off-season RSV epidemics in Australia after easing of COVID-19 restrictions. Nat Commun. 2022 May 24;13(1):2884. https://doi.org/10.1038/s41467-022-30485-3.
  3. Loconsole D, Centrone F, Rizzo C, et al. Out-of-Season Epidemic of Respiratory Syncytial Virus during the COVID-19 Pandemic: The High Burden of Child Hospitalization in an Academic Hospital in Southern Italy in 2021. Children (Basel). 2022 Jun 8;9(6):848. https://doi.org/10.3390/children9060848.
  4. Deutsche Welle. How have children fared in Germany in the latest wave of the COVID pandemic? News Article, 23/02/2022. Available at: https://p.dw.com/p/47Mns.
  5. Cohen R, Ashman M, Taha MK, et al. Pediatric Infectious Disease Group (GPIP) position paper on the immune debt of the COVID-19 pandemic in childhood, how can we fill the immunity gap? Infect Dis Now. 2021 Aug;51(5):418-423. https://doi.org/10.1016/j.idnow.2021.05.004.
  6. Meyer Sauteur PM, Beeton ML, Uldum SA, et al. Mycoplasma pneumoniae detections before and during the COVID-19 pandemic: results of a global survey, 2017 to 2021. Euro Surveill. 2022 May;27(19):2100746. https://doi.org/10.2807/1560-7917.ES.2022.27.19.2100746.

Associate Professor Matthias Maiwald, Head and Senior Consultant, Microbiology Service, Department of Pathology and Laboratory Medicine, KKH

Assoc Prof Matthias Maiwald graduated with a Doctor of Medicine from the University of Heidelberg in Germany and completed a Fellowship of the Royal College of Pathologists of Australasia (FRCPA) in Australia. He is also Adjunct Associate Professor at the NUS Yong Loo Lin School of Medicine and Duke-NUS Graduate Medical School. Assoc Prof Maiwald’s key interests include microbiological aspects relevant to women’s and children’s health, molecular detection and characterisation of fastidious and emerging pathogens, and infection control with an emphasis on hand hygiene and skin antisepsis.

Associate Professor Thoon Koh Cheng, Senior Consultant, Infectious Disease Service, KKH

Chairperson of the Hospital Infection Control Committee at KKH, Assoc Prof Thoon is also Chairman of the Ministry of Health’s National Verification Committee for Measles Elimination. Assoc Prof Thoon’s research interests include epidemiology of vaccine-preventable diseases, respiratory viral diseases, antimicrobial stewardship, infection control and adverse events following immunisation.