The first big flu season in several years has hit at least one Dunedin secondary school which has recorded daily absences reaching into triple figures.
Southern District Health Board medical officer of health Dr Susan Jack fears the boom in cases could be just the beginning of a major outbreak in the South after a couple of years relatively free of influenza.
Jack said the lack of cases of influenza over the past two years gave her concerns for the possible impacts of the disease on the community now that Covid-19 restrictions were loosened up.
"We haven't had any influenza in New Zealand for the last two years due to the Covid intervention ... now our borders are open and flu has come back, and so that means people have not got any immunity."
There had been 120 cases of influenza detected across the district in the past two weeks.
This number was likely to only be "the tip of the iceberg", she said, as only those admitted to hospital for respiratory problems were tested for the disease.
"So, just by having those numbers, it indicates that it's widespread in the community."
The best protection against a major influenza outbreak was high rates of vaccination, she said.
"This year we've had two years without any flu circulating and so we need to do a big push on vaccinations because we haven't got any natural immunity."
Otago Boys' High School acting rector Andrew King said the school had experienced "well over a hundred" absences on Monday this week, many of whom were believed to be suffering from an influenza infection.
There was also a stomach bug present in the school population and the ongoing effects of community Covid-19 transmission.
Many of the absences were due to the flu, as infections were starting to pop up late last week.
Boarders at the school hostel had also been affected and school policy was to send sick boarders home to recuperate.
King could not confirm the exact number of boarders affected, but said it would easily be into double figures.
School sports had also been affected over the weekend as a 1st XV and 3rd XV match were cancelled after illness meant the school was unable to field a team.
The school was being visited by Southland Boys' High School for interschool sports today, both schools remaining vigilant for signs of infection to ensure the sports could go ahead safely.
It was not unusual for the school to have a week or two heavily affected by flu in any given year, but this had not happened over the past two years after Covid lockdowns and restrictions cut down contacts for the spread of infectious diseases.
King said the spread of infection appeared to be happening primarily outside school.
Covid-19 prevention measures such as wearing masks, using hand sanitiser and opening windows for ventilation were still in place in the school, measures which would also help limit the spread of influenza.
"I think with the effect of going to the orange traffic light setting, the kids are socialising more outside of school, and obviously opening up sports in a bigger way is probably a big contributing factor as well."
Jack said Māori and Pasifika people over the age of 55, and all other adults over 65 were eligible to receive a free influenza jab, as were those with underlying health conditions, pregnant women and children who had been admitted to hospital with respiratory conditions.
Other people could still pay to get an influenza vaccination which was widely available from GPs and pharmacies.
A common misconception was that a person needed a stand-down period between testing positive for Covid-19, and receiving an influenza vaccination. This was not the case, Jack said.
"As soon as you've recovered, please go and get your flu jab. You can also get a Covid booster or vaccination at the same time as your flu vaccine."
Other prevention methods for respiratory illness would be very familiar to people after the past couple of years.
Hand washing, physical distancing and staying home when you are sick all helped prevent the spread of both Covid-19 and influenza, as did wearing masks.
"I know that under the orange traffic light setting it's not obligatory, but please do, going into our winter months, wear masks because it will protect you, not only against Covid, which we've still got circulating at high levels in our district ... [but also] influenza."