Some familiar viruses are back. Others are waiting in the wings

If you felt sick with sore throat, headache and runny nose at any point over the past year and a half, Covid-19 was the go-to diagnosis. But with public-health measures easing in many places and more people travelling, we can expect a variety of other respiratory viruses to reappear this autumn.

Influenza all but disappeared in Ireland last winter. And the rarity of hearing people hack and the absence of sniffling individuals in the community were proof that common-cold viruses were not actively transmitting among us.

Physical distancing and mask wearing were the reason for this dramatic change to seasonal illness patterns. But some familiar viruses are showing signs of resurgence, while others are waiting in the wings for the ideal time to re-establish themselves.

Just a single confirmed case of flu has been recorded in the Republic for all of 2021. This is 7,776 fewer cases than the same period in 2020

The return of children to school after the long summer break marks the beginning of cough-and-cold season. Coronavirus and rhinovirus spread rapidly among young children, and, judging by reports from parents and teachers, these common-cold viruses are flourishing.

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For younger children, respiratory syncytial virus can cause a serious illness. It, too, all but vanished last winter, but GPs are beginning to see a rise in cases again. The latest infectious-diseases report from the Health Protection Surveillance Centre shows 150 cases of respiratory syncytial virus were recorded here in the week ending October 9th. Wheezing, lethargy, persistent cough and difficulty breathing in an infant are all signs that medical treatment is needed.

There is some concern that the lack of exposure to respiratory syncytial virus over the past year may catch some children’s immune systems on the hop. Careful hand hygiene and other Covid-19 protective measures could be crucial in helping to protect infants from respiratory syncytial virus.

The one virus with an ongoing question mark over its intentions is influenza. Just a single confirmed case of flu has been recorded in the Republic for all of 2021, according to the Health Protection Surveillance Centre shows. This is 7,776 fewer cases than the same period in 2020. We usually get a preview of the upcoming flu season by watching Australia and New Zealand during their winter. But the Antipodes saw low flu activity again this year, probably as a result of Covid-19 lockdowns. But countries in Asia, sub-Saharan Africa and South America have seen more influenza transmission over the past few months.

Predicting flu activity this winter is a gamble. Researchers warn to expect the unexpected. "If anyone tells you they know, they don't know," the epidemiologist John Paget of the Netherlands Institute for Health Services Research, in Utrecht, told Nature magazine. But most experts agree that influenza will eventually rebound, particularly as travel restrictions wane.

Respiratory viruses each have their own patterns, and there may be interactions among them. The spread of one, for instance, may reduce the infectivity of another

Seasonal flu typically kills 290,000-650,000 people a year worldwide. But for most of 2020 and 2021 it simply vanished from much of the globe. Australia has had no seasonal influenza deaths so far in 2021, compared with between 100 and 1,200 in previous years.

The principal unknown in this year’s influenza equation is how international travel will influence its resurgence. So far the progressive lifting of social interventions to curb Covid-19 has not dented the decline in flu. Influenza has continued to circulate at low levels in the tropics, so you would expect to see a spread from there as borders reopen and international travel returns.

What if there is a resurgence in Covid-19 that corresponds with the return of influenza to pre-pandemic levels? Respiratory viruses each have their own patterns, and there may be interactions among them. The spread of one, for instance, may reduce the infectivity of another.

Whatever the outcome, it would be worrying to see rebound effects caused by a build-up of immunologically naive people in seasonal flu. It's why getting your flu vaccine this season, now available from GPs and pharmacies, is especially important. 

Read: Flu vaccine for children: Everything you need to know

mhouston@irishtimes.com