It’s called the “winter bump” - a seasonal spike in sickness when more Kiwis die than at any other time of the year.

Now, an analysis of nearly 150 years of data has shed new light on how this winter jump in deaths has changed over time.

Over the four coldest months, the Otago University research found mortality rates to be 22 per cent higher than the rest of the year.

Before Covid-19, this “excess winter mortality” accounted for about 1450 deaths per year – or about four times the national road toll.

While the cold months now aren’t as much a killer as they were a century ago – excess winter deaths made up nearly 8 per cent of annual deaths in 1920, compared with 4.5 per cent in the 2010s – some of the biggest drivers remain.

“The things that are more likely to kill people in winter, compared to other seasons, are respiratory illnesses,” study author Dr Lucy Telfar-Barnard said.

That notably included influenza – pre-Covid modelling attributed the flu to 500 deaths in New Zealand each year – and others like respiratory syncytial virus (RSV).

The study didn’t include recent years in which coronavirus – responsible for more than 1300 deaths last year – has been widely circulating, although Telfar-Barnard noted Covid-19 still wasn’t following a seasonal pattern like flu.

But the analysis did highlight sharp spikes in deaths over bad flu years - including the 1957 pandemic, 1996′s severe season and a period following the disastrous 1918 pandemic.

Telfar-Barnard added respiratory nasties weren’t the only known killers over winter.

“We also see more circulatory system deaths, like heart attack and stroke, but the reasons for that are a bit more complex.”

One was circulating viruses putting pressure on our immune system, and in turn our cardiovascular system; another was our blood thickening to cope with the cold, which could cause clotting.

“There are a range of other factors [behind winter deaths] that are a bit harder to interpret, but respiratory illness seems to be the big one.”

Ultimately, she said the analysis couldn’t pin-point any single factor behind the gradual drop in winter deaths over the last century – like the flu vaccine or healthier housing – so the study team simply put it down to higher quality of life.

Along with the long-term toll of Covid-19, the researchers were interested to tease out the impact of climate change, which meant milder winters, but more deaths from summer heatwaves.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

This article was originally published on the NZ Herald here.