Table 4. Estimations of overall population-level immunity against historical H2 and H1 pandemic viruses and the potential effect of population immunity on reproduction number among persons in Hong Kong, China*.

Virus strain	Proportion of population immune (95% CI)	Relative reduction in R0 (95% CI)	Smallest R0 needed to cause pandemic (95% CI)
A/Singapore/1/1957 (H2N2)	0.37 (0.346–0.394)	0.321 (0.295–0.348)	1.472 (1.419–1.535)
A/California/04/2009 (H1N1)	0.117 (0.098–0.14)	0.115 (0.096–0.138)	1.13 (1.106–1.16)

*Serum samples for testing antibodies to the 1957 virus were collected in 2011 and those for testing antibodies to the 2009 virus were collected in 2008–2009.