Table 2

Overall survival by Cox proportional hazards regression at survival periods 0–1, 0–2, 0–5 and 0–10 years post-surgery

Period of follow-up	Adjusted I^*		Adjusted II^†		Fully adjusted^‡
Season of surgery	HR (95% CI)	p Value	HR (95% CI)	p Value	HR (95% CI)	p Value
0–1 years after surgery
Winter	1 (reference)	0.053	1 (reference)	0.067	1 (reference)	0.052
Spring	1.07 (0.95 to 1.20)		1.06 (0.95 to 1.19)		1.07 (0.96 to 1.20)
Summer	1.09 (0.97 to 1.22)		1.08 (0.96 to 1.21)		1.12 (1.00 to 1.25)
Autumn	0.95 (0.84 to 1.06)		0.94 (0.84 to 1.06)		0.97 (0.86 to 1.09)
0–2 years after surgery
Winter	1 (reference)	0.19	1 (reference)	0.17	1 (reference)	0.43
Spring	0.99 (0.92 to 1.06)		0.98 (0.92 to 1.06)		1.00 (0.93 to 1.07)
Summer	0.99 (0.92 to 1.06)		0.99 (0.92 to 1.06)		1.01 (0.94 to 1.08)
Autumn	0.93 (0.87 to 1.00)		0.93 (0.86 to 1.00)		0.96 (0.89 to 1.03)
0–5 years after surgery
Winter	1 (reference)	0.60	1 (reference)	0.48	1 (reference)	0.96
Spring	0.98 (0.94 to 1.03)		0.98 (0.94 to 1.03)		1.00 (0.95 to 1.04)
Summer	0.98 (0.94 to 1.02)		0.97 (0.93 to 1.02)		1.00 (0.95 to 1.04)
Autumn	0.97 (0.93 to 1.01)		0.97 (0.93 to 1.01)		0.99 (0.95 to 1.03)
0–10 years after surgery
Winter	1 (reference)	0.90	1 (reference)	0.81	1 (reference)	0.92
Spring	1.00 (0.96 to 1.03)		1.00 (0.96 to 1.03)		1.01 (0.98 to 1.05)
Summer	1.00 (0.96 to 1.03)		0.99 (0.96 to 1.03)		1.01 (0.98 to 1.05)
Autumn	0.99 (0.95 to 1.02)		0.98 (0.95 to 1.02)		1.00 (0.97 to 1.04)

Estimates of season of surgery are shown among 79 658 Danish women operated for breast cancer between 1 June 1978 and 31 May 2010.
↵* Model stratified for treatment programme.
↵† Model stratified for treatment programme and age at surgery.
↵‡ Model stratified for treatment programme, age at surgery, hormone receptor status and nodal status and including the effects of tumour size and histological group.