[As pandemic restrictions ease, other respiratory viruses are
returning in unexpected ways.] [[link removed]]

WHY EASING COVID RESTRICTIONS COULD PROMPT A FIERCE FLU REBOUND  
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Nicola Jones
October 7, 2021
Nature [[link removed]]

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_ As pandemic restrictions ease, other respiratory viruses are
returning in unexpected ways. _

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The COVID-19 pandemic is continuing to have unusual and unexpected
effects on a number of respiratory diseases — some have been
quashed, others have ploughed through and still more are rebounding
off-season. These fluxes are complicating medical responses to the
pandemic, but also providing scientists with an opportunity to study
how these viruses spread.

As cold-and-flu season ostensibly starts in the Northern Hemisphere,
researchers warn to expect the unexpected. “If anyone tells you they
know, they don’t know,” says epidemiologist John Paget at the
Netherlands Institute for Health Services Research in Utrecht. Most
agree that influenza will eventually rebound, possibly fiercely, as
travel restrictions and societal interventions designed to curb the
coronavirus, such as mask wearing, wane. “Once we let our good
health practices lapse, then flu is likely to hit hard,” says Robert
Ware, a clinical epidemiologist at Griffith University in Queensland,
Australia.

Seasonal flu typically kills 290,000–650,000 people a year
worldwide. But for most of 2020 and 2021, it practically vanished from
much of the globe. FluNet, a tool for tracking global virological data
on influenza maintained by the World Health Organization, shows that
the proportion of positive flu tests has remained roughly flat since
April 2020, despite increased surveillance (see ‘Viral shift’).

Source: World Health Organization

Flu break

The United States recorded just 646 flu deaths in the 2020–21 season
— the annual average is in the tens of thousands — and there was
only one paediatric flu death. Australia has had no seasonal influenza
deaths so far in 2021, compared with between 100 and 1,200 in previous
years.

The decline in flu has persisted despite the variable lifting of
social interventions to curb coronavirus. This hammers home the
importance of international travel in bringing flu to any given
country, says Richard Webby at St. Jude Children’s Research Hospital
in Memphis, Tennessee. “It says a lot about seeding events and how
important they are,” he says. Flu has continued to circulate at low
levels in the tropics, researchers note, so will probably spread from
there once borders reopen.

Pandemic response measures also seem to have suppressed some bacterial
infections, including those that cause pneumonia and meningitis and
are associated sepsis1
[[link removed]].
But some viruses have behaved differently
[[link removed]]. Rhinoviruses,
for example, a major cause of the common cold, continued to spread
throughout the pandemic, and infections even shot up in some
countries2
[[link removed]],
possibly because these viruses aren’t as susceptible as many others
to measures such as surface cleaning and hand washing, and because
they faced little competition from other respiratory viruses. There is
emerging evidence that these mild viruses might protect people from
more serious diseases during infection3
[[link removed]],4
[[link removed]].

And some typical winter viruses have bounced back out of season.
Infections caused by common human coronaviruses (another major
common-cold culprit) and parainfluenza viruses were at very low levels
in the United States in 2020 but started to rise to pre-pandemic
levels in spring 2021 — an unusual time for colds to hit. Likewise,
infections with respiratory syncytial virus (RSV), which usually
causes mild cold symptoms but is also responsible for about 5% of
deaths in children under 5 worldwide, were at a historic low for a
year and then started rising months later than usual, in April 2021.
RSV infections were still climbing at the end of August
[[link removed]] (see
‘Strong rebound’).

Source: Foley, D. A. _et al. Clin. Infect.
Dis_. [link removed]
[[link removed]] (2021).

Off-season peaks

The strangely timed bumps might be related to school reopenings,
according to a report from the US Centers for Disease Control and
Prevention (CDC)5
[[link removed]],
along with a build-up of susceptible, unexposed infants in the absence
of a vaccine (RSV vaccines
[[link removed]] are
under development).

Off-season RSV peaks have been seen elsewhere, too, in countries
including South Africa, Japan, Australia and the Netherlands. In
Western Australia, an RSV peak in December 2020 was 2.5 times larger
than the July peak for 20196
[[link removed]].
A steep onset of disease doesn’t necessarily translate to more cases
overall, however: the total number of RSV cases in Queensland was
lower than usual, Ware notes, “but because all the cases came close
together it was much more intense”, putting strain on health
resources.

It would be worrying to see rebound effects caused by a build-up of
immunologically naive people in seasonal flu, researchers caution.
Around the world, there are signs of circulating H3N2, H1N1 and B
influenza viruses, says Amber Winn, an epidemiologist in the CDC’s
Division of Viral Diseases in Atlanta, Georgia. A wave of influenza B
infections in winter 2019–20, she notes, contributed to a record
number of paediatric flu deaths that season. “That’s why getting
your flu vaccine this season may be especially important,” she says.

_doi: [link removed]
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References

* 1.
Brueggemann, A. B. _et al._ _Lancet Digital Health_ 3, E360–E370
(2021).

PubMed
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* 2.
Takashita, E. _et al._ _Influenza Other Respir. Viruses_ 15,
488–494 (2021).

PubMed
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* 3.
Van Leuven, J. T. _et al._ Preprint at
bioRxiv [link removed]
[[link removed]] (2021).

* 4.
Dee, K. _et al._ _J. Infect. Dis._ 224, 31–38 (2021).

PubMed
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* 5.
Olsen, S. J. _et al._ _Morbid. Mortal. Weekly Rep._ 70, 1013–1019
(2021).

PubMed
[[link removed]] Article
[[link removed]] Google Scholar
[[link removed].] 

* 6.
Foley, D. A. _et al._ _Arch. Dis. Child_.
[link removed]
[[link removed]] (2021).

Article [[link removed]] Google
Scholar
[[link removed].] 

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