How worried should we be about flu this winter?

The run-up to the festive holidays last year coincided with a sudden, massive surge in influenza rates to their highest level in more than a decade.

In the space of just three weeks, from the end of November to Christmas week, there was a six-fold increase in flu infections.

Scotland's most recent nasty flu season - the winter of 2017/18 - peaked at around 46 known cases per 100,000 people, but that was exceeded last winter by a high of around 54 per 100,000.

The number of acute hospital admissions for flu hit a high of 1,339 in the week leading up to Christmas last year versus a peak of 1,139 in the final week of 2017.

All in all, last winter appeared to have been a shocker for flu.

The Herald: Flu rates in Scotland in the 2022/23 season hit an 'extraordinary' high in DecemberFlu rates in Scotland in the 2022/23 season hit an 'extraordinary' high in December (Image: PHS)

As with Covid, patients who test positive for influenza in hospital have to be isolated from other patients in a bid to prevent the spread of the virus, which can cause serious complications in the frail elderly, infants, and people with compromised immune systems.

An outbreak of flu on a ward means it will be forced to close to new admissions, limiting hospital capacity.

The surge in influenza last winter undoubtedly contributed to the NHS experiencing its worst ever winter in terms of bed shortages and A&E gridlock.

Back in January, the National Records of Scotland (NRS) also reported that the country had experienced its worst weekly death toll from flu in more than 20 years after 121 people died as a result of infections in the week beginning January 9.

READ MORE: Is Scotland heading for another winter of discontent? 

A separate report published this week by the NRS on winter mortality revealed that the total number of deaths, from all causes, registered in the four-month period from December 2022 to March 2023 was the highest in 33 years.

Of the 4,137 "excess" winter deaths (calculated by comparing the winter death toll against the average for the four-month periods before and after winter), 340 were caused by flu compared to 310 by Covid.

The Herald: Record numbers of people spent over 12 hours in A&E departments last December, leading to ambulances stacking outsideRecord numbers of people spent over 12 hours in A&E departments last December, leading to ambulances stacking outside (Image: Getty)

Last winter was certainly the deadliest in more than 30 years, then - but how deadly was the flu outbreak?

Surprisingly, despite the massive surge in cases and hospitalisations, it actually claimed fewer lives than the 2017/18 season.

According to quarterly NRS "vital events" statistics, which provide a breakdown of all registered deaths by their underlying cause, there were 378 influenza deaths in total in the six months from October 2022 to March 2023.

That compares to 386 in the same six-month period for 2017/18.

One explanation could be that the strains which circulated in the winter of 2017/18 were more virulent, resulting in more severe disease.

It could also mean that last winter's batch of flu vaccine was better matched to the virus, or that vaccine coverage was higher - especially among the highest risk groups.

In 2017/18, the flu vaccine turned out to be much less effective than usual in preventing hospitalisations for flu. 

READ MORE: Deaths in Scotland are on the rise - so what's the cause? 

It is also worth noting that while flu accounted for a larger number of excess winter deaths than Covid - 340 to 310 in December to March - looking across the full six month period from October to March shows that, in total, there were 899 Covid deaths compared to 378 caused by flu.

This reflects the fact that Covid rates remain relatively high all-year round, whereas flu is very seasonal. As a result, the spike in flu deaths during winter was more pronounced.

Looking ahead to the winter of 2023/24, then, what might we expect?

Everyone over 50 in Scotland is eligible for a winter flu vaccine, plus anyone who falls into other priority categories based on health conditions or their occupation.

So far uptake is reasonably high, at around 74%, among older care home residents (it reached 89% last winter), but low - at 20-40% - for other eligible groups. The rollout is ongoing, however.

One of the ways we can gauge how our winter flu season might to pan out is by looking at what happened in Australia, as the virus moves from the southern to northern hemisphere.

Australia's winter flu season started early again in 2023, peaking at the end of June. That is two weeks later than in 2022, but before the pandemic Australia's flu rates tended to be highest in August.

If the UK follows the same pattern we could anticipate another influenza boom some time between mid-December and mid-January.

According to the Australian Government's most recent surveillance report for influenza, there have been a total of 243,844 laboratory-confirmed influenza cases up to October 1 this year.

That is slightly more than the total of 225,332 in the full 2022 flu season, from January 1 to October 9, which was itself double the five-year average.

In that sense then we can probably expect a second winter with much higher than usual flu infections, and all the strain that brings to the NHS.

Unlike 2022, however, flu cases in Australia rose more slowly and took longer to fall away this year.

Whereas cases peaked at a record high of 30,000 in a single week in 2022, this year's peak was around 18,000.

The Herald: This year's flu season in Australia (red) has been longer with more cases overall, but avoided the extreme peak of 2022 (orange)This year's flu season in Australia (red) has been longer with more cases overall, but avoided the extreme peak of 2022 (orange) (Image: Australian Influenza Surveillance Report)

If the UK follows suit it would mean a longer but less dramatic flu season, without the sudden over-and-out surge that helped to swamp hospitals last December.

In terms of the severity, there have been 252 influenza-associated deaths registered in Australia this year compared to 308 last year which suggests that this year's case-fatality rate is slightly lower, at 0.1% instead of 0.14%.

In contrast, the number of people admitted to hospital with confirmed influenza was much higher in 2023 - at 3,349 - compared to 1,832 in 2022.

READ MORE: Hundreds of hospital wards closed due to Covid outbreaks 

This may reflect increased testing, however, as the percentage of patients who were admitted directly to intensive care due to severe flu illness was essentially unchanged, at 7.1% this year (238 patients) and 6.7% in 2022 (122 patients).

It is too early to say how effective this year's batch of flu vaccines will be, but they are the best tool we have to limit the impact of the virus on the NHS and - hopefully - avoid a repeat of 2022's "worst winter ever".