Table 1.

Description of the study population

	Season 2012–2013 n=794 138	Season 2013–2014 n=735 136	Season 2014–2015 n=660 216
Age (years)
2–3	13% (99 855)	13% (93 161)	13% (83 241)
4–8	31% (248 486)	31% (232 735)	32% (211 204)
9–17	56% (445 797)	56% (409 240)	55% (365 771)
Sex
Male	51% (406 251)	51% (375 999)	51% (337 454)
Female	49% (387 887)	49% (359 137)	49% (322 762)
Region
England/North	14% (114 636)	14% (102 314)	12% (76 849)
England/Midlands	18% (147 158)	16% (121 547)	15% (100 850)
England/South	32% (253 833)	32% (232 379)	32% (215 025)
England/London	12% (94 681)	13% (97 368)	13% (83 585)
Northern Ireland	4% (28 189)	4% (28 090)	4% (28 204)
Scotland	11% (85 423)	12% (85 931)	13% (84 993)
Wales	9% (70 218)	9% (67 507)	11% (70 710)
High-risk condition
Chronic respiratory disease (including asthma)	5% (40 052)	5% (35 561)	5% (30 484)
Chronic heart disease	1% (6892)	1% (6511)	1% (5943)
Chronic kidney disease	<1% (412)	<1% (385)	<1% (356)
Chronic liver disease	<1% (232)	<1% (213)	<1% (183)
Chronic neurological disease	<1% (3612)	<1% (3384)	<1% (3031)
Diabetes	<1% (2052)	<1% (1969)	<1% (1740)
Immunosuppression	<1% (1483)	<1% (1427)	<1% (1301)
Other high-risk condition	<1% (391)	<1% (298)	<1% (236)
Any of the conditions above	7% (53 475)	7% (48 254)	6% (41 940)