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,
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
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.
Read more: How do you tell if it’s the flu or COVID-19? Find out here.
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
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
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
For more information on the flu and Singapore’s vaccination programmes, visit
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