Even though the seasonal flu and common cold were overshadowed by the COVID-19 pandemic in the past two years, it is still important to get the seasonal flu jab to prevent co-infections and complications.

Singapore’s seasonal flu, common cold, and other respiratory viral infections saw a significant reduction in the past two years, even as the COVID-19 virus infected thousands and killed hundreds.

While the much lower numbers might make the flu jab now seem redundant, it is still needed, according to a study by Singapore General Hospital (SGH) and KK Women’s and Children’s Hospital (KKH) microbiologists.

“The response measures aimed at reducing COVID-19 transmission, such as maskwearing, hand-hygiene practices, working from home and border restrictions, had inadvertently lowered Singapore’s number of unrelated respiratory virus infections, including influenza (flu) A or B, and the common cold viruses,” said Dr Wan Wei Yee, Senior Consultant, Department of Microbiology, SGH.

However, the study noted that each time some measures were eased, flu and other viral infections returned. “Singapore may see a re-emergence of respiratory virus infections, including influenza, with the easing of travel restrictions and other pandemic control measures,” said Dr Matthias G Maiwald, Senior Consultant and Head, Microbiology Service, Department of Pathology and Laboratory Medicine, KKH.

Singapore started allowing vaccinated travel lanes (VTLs), or quarantine-free travel, in September 2021, in a cautious reopening to the outside world.

The study examined data of 42,558 polymerase chain reaction (PCR) tests from SGH, KKH and the National University Hospital in 2019 and 2020. It found that as in previous years, flu and other respiratory virus infections were common in 2019, peaking in December 2019 and January 2020, before starting to decline during the DORSCON Orange and Circuit Breaker periods in the first half of 2020.

Data for 2021, which were outside the study, indicated a similar pattern of the viruses emerging and waning according to Singapore’s stringency measures. “In 2021, influenza was basically non-existent in SGH. The pandemic measures pretty much kept the levels of flu very, very low,” said Dr Wan.

Some common cold viruses came back during the reopening phases in the second half of 2021, but “flu A and B (two of four influenza strains that cause Singapore’s seasonal flu infections) remained near absent in 2021”, said Dr Maiwald, adding that with greater travel and relaxation of measures, the flu virus will also likely return.

He pointed out that the spike in infections in December 2019 and January 2020 was typical of the pattern seen yearly (before the COVID-19 pandemic), which coincided with the school holidays when families travelled overseas.

Vaccination against the flu virus is important and necessary, especially as the dearth of flu infections in the past two years has made our immune systems less prepared for the return of infections, said Dr Maiwald. “Common cold infections keep our immunity trained,” he added.

Moreover, it is possible to be infected with both COVID-19 and the flu virus, which is known as co-infection. In studies conducted in other countries, common viruses occurring in COVID-19 patients included the flu A virus, followed distantly by flu B, said Dr Esther Tan, Consultant, Department of Respiratory and Critical Care Medicine, SGH.

No such cases have been detected so far in Singapore, but as the symptoms for both the flu and COVID-19 infections are similar, co-infections might have escaped detection, she said. “The concern with co-infection is that the symptoms may be aggravated, while complications may be more profound, resulting in poorer outcomes.”

Co-infections can lead to a higher number of hospital admissions, longer hospital stays, the need for oxygen support and mechanical ventilation in intensive care, and even death. After recovering from a viral infection, such as flu or COVID-19, the patient risks getting a secondary bacterial infection (bacteria superinfection), which again can lead to greater complications and poorer recovery outcomes.

Dr Tan, who is not a member of the study team, reiterated the government’s recommendation for the elderly, those with chronic medical conditions or on longterm care, as well as women at any stage of pregnancy, to get a yearly or seasonal flu vaccination.

A 14-day interval between the flu and COVID-19 vaccinations is necessary to avoid potential interactions.

Dr Wan and Dr Maiwald are respectively the lead author and co-author of Trends in Respiratory Virus Infections during the COVID-19 Pandemic in Singapore 2020, which was published in the JAMA Network Open journal in June 2021.

For more information on the flu and Singapore’s vaccination programmes, visit www.moh.gov.sg/covid-19/vaccinationwww.healthhub.sg/a-z/diseases-and-conditions/103/topics_influenzawww.moh.gov.sg/resources-statistics/nationally-recommended-vaccines.

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