Commentary: Should Singapore be bracing for a bad flu season?

SINGAPORE: It’s that time of year when it seems like everyone you know has been falling sick. But isn’t it “just the flu”?

Influenza activity has been on the rise across the globe since September. The United States and the United Kingdom are bracing for bad flu seasons in the current winter months, with health authorities already seeing an earlier-than-usual surge in cases. They warn of a “tripledemic” where the concurrent outbreaks of COVID-19, flu and RSV (respiratory syncytial virus) could pile on pressure to the healthcare system.

Should Singapore be bracing for a bad flu season as well? Health Minister Ong Ye Kung said on Sunday (Dec 4) that a new COVID-19 wave is expected around the end of the year.

Influenza activity usually peaks in the winter months elsewhere. In Singapore, we experience year-round circulation of influenza virus, with two prominent peaks from May to July and from December to February, coinciding with the rainy seasons.

Singapore’s influenza activity typically increases around the year-end, with the December school holidays and festive celebrations including Christmas and New Year tend to involve travelling overseas or gathering in large groups. Travelling to colder destinations in the Northern Hemisphere where influenza transmission is active could also mean people catch the flu and carry the virus when they return.

The reality is that seasonal influenza is associated with 290,000 to 650,000 deaths globally every year. Studies have shown that hospitalisation rate in Singapore is 50.1 per 100,000 person-years, but higher rates are found in young children and the elderly aged over 65 years.

RESPIRATORY ILLNESSES BACK WITH A VENGEANCE

Experts have been warning that common respiratory illnesses could return with a vengeance as pandemic measures are lifted. Strategies, including aggressive testing, travel restrictions, lockdowns and mask wearing, were effective in controlling the transmission of not only the coronavirus but also influenza and other respiratory viruses such as RSV.

The number of influenza cases dramatically dropped during the COVID-19 pandemic, leading to almost zero circulation of human influenza viruses for more than two years. Together with lower rates of influenza vaccination, this likely resulted in waning population immunity, making all of us more susceptible as influenza strains resume global circulation.

Another reason could be due to the dominant H3N2 subtype this time round. The burden of influenza varies from season to season, depending on the prevalent strain. At present, there are four subtypes of influenza viruses circulating among us: Two subtypes of influenza A (H1N1 and H3N2) and two of influenza B (Victoria and Yamagata).

Of the four, H3N2 viruses typically have a faster rate of mutation. Similar to SARS-CoV-2 virus, influenza viruses are RNA viruses that mutate with time and generate new variants.

Like the coronavirus and its spike protein, influenza subtypes are characterised by the hemagglutinin protein (such as H3 in H3N2) which binds to the cell to cause infection. Mutations in this protein can enable the virus to evade our immune system.

This means the H3N2 strain in the influenza vaccine will need to be updated more frequently or risk becoming less effective against the current circulating viruses.

EFFECTIVE VACCINES AGAINST INFLUENZA

The good news is that there are effective vaccines against influenza, the so-called quadrivalent vaccines, as they contain - and thus confer protection against - the four subtypes, whichever becomes the dominant one each season.

But given the time needed to manufacture the vaccines, the World Health Organization consults global scientific experts to determine the strains that will make up the vaccine for the upcoming flu season – twice a year to recommend the composition for vaccines in the Northern and Southern Hemispheres. How well-matched the vaccine strains are to the circulating viruses affects the vaccine effectiveness.

This season, the prevalent H3N2 virus belongs genetically to a group or clade called 3C.2a1b.2a2. Current vaccines for both the 2022-2023 Northern and 2023 Southern Hemispheres influenza seasons contain a H3N2 strain of this clade and should provide protection against the circulating viruses – no matter which flu vaccine your doctor stocks.

As we hit the holiday and peak travel seasons, flu shots can be taken around two weeks ahead of travel plans to allow our bodies to develop the necessary immune response.

HOW ABOUT MORE INFLUENZA TESTING?

The pandemic has shone the spotlight on diagnostic testing as a key tool to fight COVID-19 and normalised self-testing. Given similar antigen rapid test (ART) kits exist for influenza, could doing more influenza testing help?

With COVID-19 now treated as endemic, incorporating influenza diagnostic testing would be useful to differentiate if the sick person is infected with flu, SARS-CoV-2 or neither.

Knowing what virus can help the doctors to make early precisions in prescribing suitable antiviral medications for treatment, as some anti-viral influenza drugs work best within 48 hours of symptom onset. In addition, early diagnosis and treatment can help to reduce the health burden including complications of diseases and rates of hospitalisation.

It comes down to cost and practicality. Who would bear the costs of self-test kits? Is there a need to test everyone who shows signs of respiratory illness? For a healthy individual who is up to date with their flu and COVID-19 vaccinations, the illness would be generally mild and self-limiting if infected with either virus.

Testing could be better targeted at highly vulnerable groups who risk severe clinical outcomes if infected with influenza, including young children, the elderly, pregnant women, immunocompromised individuals and people with chronic medical conditions.

TOWARDS A COMBINED COVID-FLU SHOT

The next key question will be whether different rules should be applied for COVID-19-positive versus flu-positive cases.

Currently, people with COVID-19 are told to stay home for at least 72 hours. No such instructions are given to people with flu – though the hope is they are responsible enough to stay home or at least mask up after almost three years of the pandemic-heightened vigilance.

If no additional preventive measures are needed whether catching flu or COVID-19, do we need to continue testing every sick individual? From a scientific point of view, early diagnostic and detection are indispensable for monitoring any emerging novel variants and early signs of outbreaks, so that additional preventive measures can be implemented quickly.

Influenza and SARS-CoV-2 will continue to circulate and evolve, and new variants will emerge. Currently, the flu and COVID-19 vaccines can be taken at the same visit. A combined COVID-flu vaccine could be ideal – something Pfizer and BioNTech have started studying but will need further trials to address questions about safety, side effects and effectiveness.

Ultimately, taking a yearly flu shot is a very good idea to protect ourselves and prevent severe illness, in addition to the good habits we’ve developed like washing our hands, wearing masks or staying home when unwell.

Dr Yvonne Su is an Associate Professor in the Emerging Infectious Diseases Programme at Duke-NUS Medical School.