Response to “It’s not just the snow; it’s also the cold”
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* Nathalie Auger
* Brian J. Potter
* Audrey Smargiassi
* Marianne Bilodeau-Bertrand
* Tom Kosatsky

We respond to the recent letter to *CMAJ* from Liu and colleagues.1 Similar to our results, Liu and colleagues found an association between snowfall and admission to hospital for myocardial infarction (MI) with ST-segment elevation.2 However, they also found that cold temperature was associated with admission for MI, whereas we did not. In detailed sensitivity analyses adjusting for snowfall, we found no evidence of an association between temperature and MI (Appendix 5).3 Although comparability of diagnostic categories and possibly climate may explain the results, a difference in study design seems a strong possibility. Liu and colleagues analyzed count data using an ecologic method. As outlined in our paper,3 the results from such approaches should be interpreted with caution because of the risk of ecologic bias owing to an inability to account for temporal trends.4 Consider the possibility that both temperature and incidence of MI hospital admission changed over time for unrelated reasons.5,6 Negative binomial regression — an ecologic method — would suggest an association, and be unable to distinguish between unrelated temporal trends and a true association. For this reason, we would be very interested to see a reanalysis of the St. Boniface data2 that capitalizes on individual rather than aggregate data, and better adjusts for temporal trends in weather or incidence of MI over time. We agree with Liu and colleagues that increased public awareness and possibly reorganization of health services for predictable seasonal changes in MI events are worthy of consideration.

## Footnotes

*   **Competing interests:** None declared.

## References

1.  Liu S, Ducas R, Hiebert B, et al. It’s not just the snow; it’s also the cold [letter]. CMAJ 2017;189:E546.
    
    [FREE Full Text](http://www.cmaj.ca/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiRlVMTCI7czoxMToiam91cm5hbENvZGUiO3M6NDoiY21haiI7czo1OiJyZXNpZCI7czoxMToiMTg5LzE0L0U1NDYiO3M6NDoiYXRvbSI7czoyMjoiL2NtYWovMTg5LzE0L0U1NDcuYXRvbSI7fXM6ODoiZnJhZ21lbnQiO3M6MDoiIjt9) 

2.  Liu S, Ducas R, Hiebert B, et al. How cold is too cold: the effect of seasonal temperature variation on risk of STEMI [abstract]. Eur Heart J 2015;36(Suppl): 709–10.
    
    

3.  Auger N, Potter BJ, Smargiassi A, et al. Association between quantity and duration of snowfall and risk of myocardial infarction. CMAJ 2017;189:E235–42.
    
    [Abstract/FREE Full Text](http://www.cmaj.ca/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6NDoiY21haiI7czo1OiJyZXNpZCI7czoxMDoiMTg5LzYvRTIzNSI7czo0OiJhdG9tIjtzOjIyOiIvY21hai8xODkvMTQvRTU0Ny5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30=) 

4.  Webster TF. Bias magnification in ecologic studies: a methodological investigation. Environ Health 2007;6:17.
    
    [PubMed](http://www.cmaj.ca/lookup/external-ref?access_num=17615079&link_type=MED&atom=%2Fcmaj%2F189%2F14%2FE547.atom) 

5.  1.  Pachauri RK, 
    2.  Meyer LA, 
    3.  Barros VR, 
    4.  et al
    
    ., editors. Climate change 2014: synthesis report. Contribution of working groups I, II and III to the fifth assessment report of the intergovernmental panel on climate change. Geneva: Intergovernmental Panel on Climate Change; 2014:1–151.
    
    

6.  Moran AE, Forouzanfar MH, Roth GA, et al. The global burden of ischemic heart disease in 1990 and 2010: the Global Burden of Disease 2010 study. Circulation 2014;129:1493–501.
    
    [Abstract/FREE Full Text](http://www.cmaj.ca/lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6MTQ6ImNpcmN1bGF0aW9uYWhhIjtzOjU6InJlc2lkIjtzOjExOiIxMjkvMTQvMTQ5MyI7czo0OiJhdG9tIjtzOjIyOiIvY21hai8xODkvMTQvRTU0Ny5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30=)