Table.

Cohort analyses with hypothetical figures to show the effect of selection bias

		People with Bell's palsy	People without Bell's palsy	Subtotal	Bell's palsy rate	Rate ratio
All people eligible for vaccination (n=200000)
Subgroup A* (n=100 000)
	Vaccinated	9	29 991	30 000	0·03%	1·00
	Unvaccinated	21	69 979	70 000	0·03%	..
Subgroup B† (n=100 000)
	Vaccinated	21	69 979	70 000	0·03%	1·00
	Unvaccinated	9	29 991	30 000	0·03%	..
Subgroups A and B combined (n=200 000)
	Vaccinated	30	99 970	100 000	0·03%	1·00
	Unvaccinated	30	99 970	100 000	0·03%	..
Subgroup A (all patients with Bell's palsy captured)
Vaccinated		30	29 991	30 021	0·10%	2·33
Unvaccinated		30	69 979	70 009	0·04%	..

^*Subgroup A: patients who presented to emergency rooms or hospital wards, comprising a higher proportion of older people (≥65 years) with a lower overall vaccination rate of 30%.

^†Subgroup B: other eligible people, who are relatively younger, with a higher overall vaccination rate of 70%.