Third time unlucky for flu’s return in winter?

Take both of them – the flu vaccine and COVID booster shot – that is the best way for us to prevent viral infections

No cases of influenza were detected in the community in winter 2021, but New Zealand’s lucky streak was marred by record cases of children being hospitalised with respiratory syncytial virus.

RSV is believed to have returned courtesy of the trans-Tasman travel “bubble”, raising the question of whether the wider border opening next year will bring flu back for the first time in three winters.

ESR virologist Sue Huang and Immunisation Advisory Centre director and specialist GP Nikki Turner both admit they can only do a spot of crystal ball-gazing.

Professor Turner, of the University of Auckland, says there is the potential for a heavy flu season after two years of no “wild boosting” to Kiwis’ immunity via circulating flu.

“But we just don’t know.”

Both she and Dr Huang say New Zealand needs to prepare for a tough season and watch carefully what happens with influenza in the northern hemisphere this winter.

The COVID-19 pandemic saw flu drop away in the last northern winter.

The European Influenza Surveillance Network reported in March that, of the 25,000 sentinel flu tests done in the 2020/21 season, only 0.1 per cent were positive, compared with an average of 38 per cent in previous seasons.

Dr Huang says COVID-19’s transmissibility has made it outgun flu in the respiratory virus wars. One person infected with the Delta variant, on average, infects six more people, while someone with the flu infects 1.3 others.

She says flu is also more sensitive to public health measures, leading to the unusually low levels that have been circulating worldwide.

Late last month, the US Centers for Disease Control and Prevention and the UK Health Security Agency were reporting that COVID-19 remained the dominant respiratory illness this winter and flu levels were still low (1.5 per cent positive in US and 0.6 per cent in the UK).

New Zealand, however, had only six detections after the border closed last year, and 14 this year. All cases were returnees at managed isolation and quarantine facilities, says Dr Huang.

So, while our flu vaccination rates “were really, really good”, the absence of flu circulating for two winters has reduced the country’s other pillar against flu – natural immunity.

“Natural flu infection provides a significant amount of immunity to our population,” says Dr Huang.

Her ESR research team tested a group of randomly selected, unvaccinated Aucklanders during the moderate 2015 flu season and found a third had had a flu infection – particularly the children.

Most were asymptomatic or had mild symptoms.

Dr Turner says children can get hit hard by flu, but RSV is different, affecting babies and toddlers exposed for the first time particularly severely.

Concerns about the potential impact of flu next winter on unexposed children saw paediatrician Cameron Grant earlier this year call for the flu vaccine to be added to next year’s childhood vaccination schedule.

Dr Huang says flu is always unpredictable, but there is a real risk of a significant outbreak in 2022, with the double whammy of COVID-19 infection in the community. “Even with one virus you have difficulty to cope, but with two viruses – it will make the situation much worse.”

She will be advocating for her family to have the flu vaccine as soon as it’s available, usually in April or May. For the many New Zealanders who got their COVID-19 vaccines in October/ November, this will coincide with eligibility for the COVID-19 booster shot. “Take both of them – that is the best way for us to prevent viral infections,” says Dr Huang.

Theoretically, COVID-19 could make a person susceptible to viral infection with flu as well, she says.

In normal years, respiratory surveillance finds people with co-infections, for example, the flu and RSV or flu and rhinovirus.

ESR has access to a multiplex assay that can test for flu and SARSCoV-2; hospital laboratories can also monitor for both.

But Dr Huang is not sure what the policy will be for population COVID-19 testing: “It will depend on what is prevalent at the time.”

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