But the virus can also cause severe infection in babies - especially premature infants - along with the elderly and those with weak immune systems.
ESR public health physician Dr Sarah Jeffries said young children were still the most affected, with the latest data showing around 62 cases per 100,000 of population among children under the age of 1, compared with 0.8 cases per 100,000 among people over 65.
"While these rates remain high, they are returning to expected seasonal levels following the very high peak three weeks ago just prior to the school holidays."
Across Auckland, hospitalisation rates had fallen over the past two weeks, and the level of Sari-related cases in hospitals across all ages were now sitting below the seasonal baseline threshold, and lower than expected for this time of year.
Scientists suspect many children have been paying an "immunity debt" to RSV after last year's lockdown cut transmission and squashed down rates by 98 per cent – as also happened with influenza and other seasonal nasties.
But it remained less clear how this year's ordeal arose.
Because Australia had earlier recorded RSV outbreaks – and New Zealand's explosion in cases coincided with the opening of quarantine-free travel – scientists strongly suspected the virus had been re-seeded from across the Tasman.
In one study just published online, researchers reported how RSV had also been squashed in Australia by Covid-19 interventions, before unprecedented outbreaks began there late last year and continued through the summer.
By sequencing the genomes, or genetic make-up, of RSV samples, the researchers learned that the virus had lost nearly all of its genetic diversity – save for two distinct clades, or groups.
When Covid-19 measures were relaxed, one clade circulating in New South Wales and ACT spread to Victoria, where it caused extensive outbreaks and hospitalisations early this year.
In New Zealand, scientists didn't yet have enough genomic data to conclusively draw a link to here.
ESR and Otago University evolutionary virologist Dr Jemma Geoghegan also pointed out that, because the only other comparable global dataset for RSV in 2020 and 2021 happened to come from Australia, it could easily look like the virus had travelled here solely from there anyhow.
"So we have to be cautious in how we interpret this data. Still, we're hoping to learn more about the evolution of this virus, and how it's reintroduced."
She said another hypothesis was that New Zealand's outbreak had similarly been ignited from low levels of its own circulating virus.
"We have some samples from 2019 and 2020, so we'll sequence those and then compare them with what's been circulating recently to see whether it might have been a homegrown strain that's reignited because of the winter season," Geoghegan said.
"We had so few cases last year that I would have assumed that it would've been an importation - but we will see.
"If there's a lot of genetic diversity between 2020 and 2021, for instance, that would suggest there have been lots of different importations from the global population, that's come in through Australia or MIQ.
"But if we find we're dealing with just those dominant strains that are circulating in Australia, then we should be able to confidently draw the conclusion that it's come over here from there."
