Has the COVID pandemic driven a flu strain extinct?

According to two studies from the Doherty Institute in Melbourne, one of the four influenza virus types that every year circulate among humans might have gone extinct thanks to the COVID-19 pandemic.

The studies, one published in Nature Reviews Microbiology in September and one under review, show that the influenza B/Yamagata virus has not been detected since April 2020 anywhere in the world.

Dr Marios Koutsakos, a researcher at the department of microbiology and immunology at the Doherty Institute and one of the authors of the published study, says that it’s highly unusual that B/Yamagata has not been detected in over 18 months. “It’s difficult to say with certainty, but that indicates it might have gone.”

But he said B/Yamagata could still be present somewhere in the world at extremely low levels.

The COVID-19 pandemic has driven global influenza cases to a record low. Before the pandemic, Australia recorded an average of 100,000 flu cases, with a few hundred deaths every year. In 2019, Australia saw one of the worst flu seasons in recent times, recording over 300,000 cases and more than 900 deaths.

But from the start of this year, the National Notifiable Diseases Surveillance System has recorded only 550 notifications and no deaths.

Every year, seasonal flu emerges in east and Southeast Asia and is transmitted from person to person through coughs, sneezes and breathing. As people travel across the globe, the virus is carried from place to place, hitting the northern and southern hemispheres during winter.

Limited movement of people across the globe and quarantine systems in place in most countries have broken the cycle, stopping the flu from spreading globally.

“Border closure has probably had the biggest impact on global spread,” says Koutsakos.

In addition, the broad range of measures taken to control the COVID-19 pandemic – social distancing, masks, shutting non-essential indoor venues, hand hygiene – has prevented significant flu outbreaks within countries.

Two main types of influenza, A and B, cause seasonal epidemics every year. Each includes two subtypes. A/H3N2 and A/H1N1 jump from humans to animals, causing pandemics such as the swine flu or the bird flu. B/Victoria and B/Yamagata don’t have an animal reservoir.

Case numbers have fallen for all influenza virus types, but the Melbourne researchers think that the B/Yamagata lineage might have disappeared altogether.

Because B/Yamagata only circulated among humans, Koutsakos says it hasn’t evolved much over the past decade, and humans have developed high levels of immunity against it, making the lineage particularly vulnerable.

“The vaccine component for the [B/Yamagata] lineage hasn’t changed in about seven or eight years,” he says. “It’s possible that a large part of the population has built up immunity to that lineage, driving it to extinction.”

Researchers have long thought that high vaccination coverage would eventually eradicate B/Yamagata, and the COVID-19 pandemic might have accelerated the process.

But as countries begin to reopen their borders and influenza recommence to spread across the world with eager travellers, we should not forget the lessons learned from the COVID-19 pandemic.

Adequate ventilation in public buildings must remain a priority. As Geoff Hanmer, an architect and adjunct professor of architecture at the University of Adelaide, told Cosmos, we don’t need to accept getting flu and colds as a normal part of winter. Respiratory viruses could be controlled by making sure people have access to clean air.

Good hygiene and mask-wearing in crowded places are good habits we should keep, Koutsakos says, and a high level of vaccination next winter could whip B/Yamagata once and for all.