COVID-19 has suppressed influenza for now, but it will return with greater severity: Study

COVID-19 appropriate behaviour will delay competition among existing influenza lineages which will eventually expand, compete and once again circulate more widely

By Taran Deol
Published: Tuesday 05 April 2022

Global influenza circulation has been heavily suppressed. Photo: Meeta Ahlawat / CSEMeasures taken around the world to curb COVID-19 had another positive effect: They curbed the circulation of influenza. But the virus may return with a vengeance, a recent study has warned.

Influenza cases peaked in the northern hemisphere during winter to 40,000-60,000 per week in 2018-2019 and 2019-2020. The figure dropped dramatically to less than 100 cases per week in May and remained so till September 2020 — a 99.8 per cent reduction, the study said.

There are two main types of influenza virus that infect humans — Influenza A and Influenza B, according to the United States’ Centers for Disease Control and Prevention (CDC). The former has two subtypes — A (H3N2) and A (H1N1) — while the latter has two lineages — B/Yamagata and B/Victoria lineage.

The influenza B/Yamagata lineage has not been conclusively detected since April 2020, while the A (H3N2) subtype, A (H1N1) subtype and B/Victoria lineage have been circulating with “considerably less genetic diversity,” according to a study, the outcome of which was published in the Nature journal March 31, 2022.

The study was carried out by scientists from the World Health Organization Collaborating Centre for Reference and Research on Influenza as well as public health experts from the University of Hong Kong.

A (H3N2) outbreaks have largely been restricted to South and Southeast Asia, while China has seen outbreaks of B/Victoria, with A (H1N1) circulating in West Africa.

The reduced cases persisted, increasing only marginally to 200-400 per week in early 2021. A similar trend was recorded in the southern hemisphere, where weekly cases would hover between 1,500 and 3,500 in 2017-2019. Less than 12 weekly cases were reported between May 2020 and July 2021.

“Prolonged suppression of seasonal influenza virus circulation, compounded by regional inequities in vaccine distribution and potential vaccine complacency, supply chain disruptions and misinformation amid fewer cases, will reduce population immunity and increase severity of future influenza virus epidemics,” the study noted.

Changes in influenza lineage circulation

Source: Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination, Nature, March 2022

There are two key ways in which the influenza virus changes. One is ‘antigenic drift’ which essentially means small changes that happen continuously over time. This is the reason why people need annual flu shots to keep them protected.

The other is ‘antigenic shift’, when a drastic change happens — this is a more rare occurrence.

“Type A viruses undergo both antigenic drift and shift and are the only flu viruses known to cause pandemics, while flu type B viruses change only by the more gradual process of antigenic drift,” the CDC notes. The Nature study found that the virus diversity has reduced, with B/Yamagata nearly extinct.

Only 2,521 influenza A (H3N2) virus cases from 57 countries have been reported between April 2020 and July 2021, the majority of which have been from India (445), Laos (268), Bangladesh (209) and Pakistan (162).

The majority of A (H1N1) cases were reported from Ghana (235), Togo (226), the United States (170) and Russia (165) during the same time period.

B/Victoria cases were largely reported from China, with some of its clades being frequently recorded in Kenya, the United States, Brazil and Japan as well.

The COVID-19 pandemic is now in its third year and vaccination drives are well underway. Consequently, several countries have begun letting go of restrictions such as social distancing, mask mandates and travel restrictions.

“We speculate that heterogeneity in COVID-19 vaccination rates and control policies will slow the global resurgence of influenza, delay competition among existing influenza lineages and enable further divergence of spatially separated lineages, but these individual influenza lineages will eventually expand, compete, and once again circulate more widely,” the paper concluded.

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