New Zealand is still virtually flu-free, monitoring shows – something a virologist says could pose a health system headache alongside Covid-19 when our borders eventually re-open.

Normally each year, more than 200,000 New Zealanders catch influenza and an estimated at least 500 people die from it – a figure equivalent to 2 per cent of all deaths, and more than the annual road toll.

Last year, however, flu rates were dramatically crushed by 99.9 per cent due to the circuit-breaking effects of closed borders, a nationwide lockdown, record flu vaccinations and other measures to stop Covid-19.

ESR virologist Dr Sue Huang said that trend has continued over this year’s winter period, with no influenza epidemic or outbreak reported, and almost no flu circulating in the country.

Since January 1, only five cases have been confirmed by ESR – mainly among travellers returning from overseas.

Cases of other respiratory viruses and infections spreading around the country have also been squeezed down to low rates by New Zealand’s second lockdown.

The most recent report from the WellKiwis study, which has been tracking flu among children in the Wellington region since 2019, showed how cases of rhinovirus, which causes the common cold, and other viruses plummeted between August and September.

Rates of respiratory syncytial virus (RSV) – which drove a national outbreak this year – fell from about 40 Wellington cases over a week in July, to none at the latest count.

Nationally, ESR weekly reporting showed weekly RSV cases had tumbled from nearly 1000 at mid-winter, to just over 40 earlier this month.

Scientists have suggested the earlier RSV surge likely stemmed from the re-opening of transtasman travel – Australia had battled an off-season outbreak in February and March – and the fact that last year’s lockdown had left children with little natural immunity.

Otago University virologist Dr Jemma Geoghegan said the reintroduction of flu into New Zealand, when global travel restrictions eased, could come with similarly severe consequences.

One study by Australian and Hong Kong researchers, and just published ahead of peer review, found that influenza strains were continuing to perish globally, under pressure of Covid-19 measures – but the virus still circulating in select hotspots would likely seed future epidemics.

It indicated that heterogeneity in Covid-19 vaccination rates and ongoing public health interventions would likely slow the global resurgence of individual influenza lineages, thereby delaying the potential for competition among those still existing.

“Upcoming influenza seasons could therefore be compounded in severity as immunity wanes over time for all age groups,” the study authors said.

Geoghegan added that, like RSV, children would’ve had little prior exposure to the virus, and thus little protective immunity – while those under two wouldn’t have had any at all.

“So it’s likely to be very severe, and it also has consequences for trying to predict how to make up a vaccine,” she said.

“Each year, a certain amount of strains go into making up the vaccine and usually, they’re predicted based on circulating strains.

“Due to a lack of information, it’d be really hard to construct a vaccine that’s effective.”

She added that flu wouldn’t be the only problem facing our healthcare system in a reopened New Zealand, with multiple infectious diseases – Covid-19 likely among them – all adding to the strain.

Meanwhile, Huang said it was important that Kiwis experiencing any flu-like illnesses now follow public health advice – including getting tested for coronavirus.

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