The researchers speculate that one reason heart-related deaths rise at Christmas is that people may delay seeking medical attention during the holidays.
The researchers - including Josh Knight, research fellow at the University of Melbourne in Australia - report their findings in the Journal of the American Heart Association.
Knight explains the reason for their study, which revisits an earlier analysis of data from the United States that found cardiac deaths are higher around Christmas and New Year than at any other time:
"However, the Christmas holiday period (December 25th to January 7th) in the U.S. falls within the coldest period of the year, when death rates are already seasonally high due to low temperatures and influenza."
Knight and colleagues used data from New Zealand, where Christmas occurs during the summer months and when death rates tend to be lower than at other times of the year.
For their analysis, the team used New Zealand Ministry of Health data from 1988-2013. Over the 25-year period, there was a total of 738,409 deaths - including 197,109 reported as cardiac deaths.
Spike in heart-related deaths outside hospital
From the records, the researchers were able to analyze deaths in hospitals separately from deaths occurring outside of hospitals.
Using similar statistical methods as the previous U.S. study, the team calculated an "expected" number of deaths for each day of the year and compared it with the actual number of deaths.
The analysis found a 4.2 percent spike in heart-related deaths occurring outside of a hospital over the Christmas period - that is, from December 25th to January 7th.
It also found that the average age of people who died during the Christmas period was slightly lower - 76.2 compared with 77.1 years at other times of the year.
Although they discuss possible reasons for this in their paper - much like the investigators of the previous study - the researchers did not investigate the causes of this apparent "holiday effect."
Possible explanations for 'holiday effect'
Potential reasons discussed to explain the holiday effect include:
- Change in diet
- Alcohol consumption
- Emotional stress
- Under-staffing in hospitals
- Visiting relatives
- Other changes in the physical environment
Another reason, suggest the researchers, is that people may delay seeking medical attention during the holiday period.
Knight also mentions that travel could be a factor. For instance, New Zealanders often holiday away from home over Christmas, and they may find themselves far from their own doctor and could be unfamiliar with the local medical facilities.
Some have suggested that heart-related deaths spike at Christmas because of increased pollution - which goes up in many parts of the world in winter because of increased heating. This is not a factor in New Zealand during the summer.
Another explanation could be what the researchers refer to as "displacement of death," during which terminally ill people, for whatever reason, hold off death for a day that is important to them.
"The ability of individuals to modify their date of death based on dates of significance has been both confirmed and refuted in other studies, however it remains a possible explanation for this holiday effect."
The researchers note that another method of estimating expected deaths would need to be used to fully explore whether the displacement effect - which concentrates deaths during the holiday period as opposed to adding extra deaths - is a significant factor.