Resumption of international travel will likely mark the return of influenza: Experts

SINGAPORE: The resumption of international travel and open borders amid loosening COVID-19 restrictions is likely to mark the return of other respiratory infections like influenza, said infectious diseases experts. 

Most respiratory viruses started to decline when Singapore moved to DORSCON orange in February 2020, and became nearly absent during the “circuit breaker” from Apr 7 to Jun 1 last year, with the strictest pandemic measures in effect, said Dr Matthias Maiwald, a microbiologist with KK Women's and Children's Hospital (KKH). 

The Disease Outbreak Response System Condition (DORSCON) level guides Singapore's approach to outbreaks like COVID-19. 

The colour-coded system - which has Green, Yellow, Orange and Red categories - shows the current situation. It also indicates what needs to be done to prevent and reduce the impact of infections.

A study conducted by Dr Maiwald and Dr Wan Wei Yee, a microbiologist with the Singapore General Hospital (SGH) showed that in Phase 1 and Phase 2 of Singapore’s opening between June and December last year, some viruses “came back”, but influenza remained almost totally absent throughout this year. 

“Our assessment is that the respiratory infections will return sooner or later when the measures are lifted. They will likely return when wider travel resumes, we don’t quite know when it will return but we predict it will return,” said Dr Maiwald. 

The National University Hospital (NUH) saw zero cases of influenza this year, said Professor Paul Tambyah, senior consultant with its Division of Infectious Diseases. 

Quoting Ministry of Health (MOH) reports on influenza surveillance, Prof Tambyah noted that Singapore had one case of influenza B in July 2021, one in April 2020 and zero cases of influenza A since March 2020. 

“As there were zero cases of influenza, there was zero influenza death,” said Prof Tambyah, who is also a Professor of Medicine at the National University of Singapore’s (NUS) Yong Loo Lin School of Medicine.

However, the reasons why influenza has disappeared in most of the world while other viruses have reappeared are “unclear”, he added, noting that this was observed in NUH and across other SingHealth hospitals. 

“No one has a good explanation for this. There are various theories including - cross-protection between influenza and coronaviruses, or possibly different modes of transmission between the different respiratory viruses.” 

The main reasons for the sharp drop in influenza cases are COVID-19 measures - mask-wearing, increased attention to hand hygiene, social distancing and the shutdown of international travel, said Associate Professor David Lye of Nanyang Technological University’s (NTU) Lee Kong Chian School of Medicine. 

The return of influenza depends on these measures moving forward, he added. 

“It is expected that once most countries open up there will be a resurgence of influenza,” he added. 

“From our study, we did hypothesise that the … lack of travel was quite strongly associated with the downfall of influenza in the DORSCON orange phase,” said Dr Maiwald.  

“So the reverse conclusion could be true, that when international travel recurs, influenza will come back. That is a reasonable construction.” 

A MORE SEVERE FLU SEASON

Safe management measures were “really effective” at reducing respiratory infections like influenza, said Professor Teo Yik Ying of the NUS Saw Swee Hock School of Public Health. 

However, it is “premature” to discuss deaths from influenza after the COVID-19 pandemic since this depends on whether safe management measures will continue, he added. 

It also depends on whether the deaths that would “normally be attributable” to influenza would be split between COVID-19 and influenza, said Prof Teo. 

In this scenario, Singapore could see slightly lower numbers of deaths from influenza, but a generally higher number of deaths when considering both diseases, he added. 

It is also “possible” for Singapore to see a more severe influenza season as the COVID-19 pandemic fades, said Dr Maiwald. 

“This is possible but strictly speaking, it is not known. We do not have data, but there is such a possibility,” he added. 

One “nightmare scenario” after the pandemic is over could be the emergence of a new strain of influenza in early-2023, since the world has not seen influenza for “such a long time”, said Prof Tambyah. 

For example, this may be a new subtype of influenza A, like H2N1, or a re-emergence of H2N2, which disappeared in 1968, he noted. 

“And we will be back to square one with the next pandemic. We will have to go through the whole process we went through with each pandemic from 1918, 1957, 1968, H1N1 2009 and now the COVID-19 pandemic all over again from epidemic to pandemic to endemic,” said Prof Tambyah. 

A novel strain or re-emergent strain of influenza will result in more deaths among young people than older folks, like in the 1918 pandemic, he added. 

This is also similar to what happened with the H1N1 outbreak in 2009, said Prof Tambyah. 

"The older people who had lived through the 1918 influenza A H1N1 pandemic paradoxically did not have as severe disease as some of the younger people, especially pregnant women.

"This applies more to re-emergent strains than a totally new strain, although the experience of countries who have had quite severe disease in children with H5N1 is not reassuring about novel strains of influenza." 

The resulting outbreak could even be worse than the current pandemic, Prof Tambyah warned, noting that influenza has "some similarities" with COVID-19. 

"It spreads rapidly, can be infectious before the onset of symptoms and vaccines are not that effective in prevention," he added. 

People are currently protected from severe seasonal influenza "to a certain extent" because of previous vaccination or exposure to circulating H1N1 and H3N2 strains, he noted. 

"If there is a new strain or a re-emergence of the 1957 H2N2, there is high risk that there will be a rapidly spreading pandemic that could have a much higher mortality rate than the 2 per cent reported globally for COVID-19," said Prof Tambyah. 

While there are no overseas studies yet that show that a year with fewer influenza infections would then cause a “rebound”, the concern still remains that the risk is higher if individuals are exposed to the influenza virus and spreads it in the community, said Dr Esther Tan, a respiratory physician with SGH. 

The experts who conducted the study urged individuals to not only get vaccinated for COVID-19, but also influenza.

“For individuals 65 years and older we definitely recommend that you do get vaccinated. However, for the individuals that are younger than 65 years old, it is recommended for those that have specific medical conditions or indications,” Dr Tan noted. 

This could include those with chronic disorders of the lung or heart, or recurrent hospitalisations due to chronic diseases like diabetes and kidney or kidney problems, she added.