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Table 3 Estimated absolute risks of severe outcomes per infection

From: Relative incidence and individual-level severity of seasonal influenza A H3N2 compared with 2009 pandemic H1N1

Outcome Strain Rounds Infectionsa Estimated severe outcomesb Risk per 10,000 infections (Binomial 95% CI)c
Excess all-cause deaths A(H1N1)pdm09 1 to 2 655000 61 0.93 (0.66 , 1.31)
2 to 3 520000 160 3.08 (2.09 , 4.55)
3 to 4 971000 359 3.70 (2.81 , 4.88)
A(H3N2) 1 to 2 158000 73 4.62 (2.27 , 9.30)
2 to 3 520000 409 7.87 (5.35 , 11.6)
3 to 4 67800 37 5.46 (1.89 , 14.9)
Excess respiratory deaths A(H1N1)pdm09 1 to 2 655,000 57 0.87 (0.62 , 1.23)
2 to 3 520,000 130 2.50 (1.70 , 3.70)
3 to 4 971000 289 2.98 (2.27 , 3.93)
A(H3N2) 1 to 2 158000 34 2.15 (1.06 , 4.33)
2 to 3 520000 190 3.66 (2.48 , 5.40)
3 to 4 67800 17 2.51 (0.87 , 6.83)
Excess respiratory hospitalisations A(H1N1)pdm09 1 to 2 655000 3470 53.0 (37.7 , 74.8)
2 to 3 520000 2480 47.7 (32.4 , 70.5)
3 to 4 971000 4590 47.2 (36.0 , 62.4)
A(H3N2) 1 to 2 158000 942 59.6 (29.3 , 120)
2 to 3 520000 5300 102 (69.3 , 151)
3 to 4 67800 476 70.2 (24.3 , 191)
  1. aEstimated cumulative number of seroconversions between study rounds, adjusted for the difference in the age distribution between study participants and the overall Hong Kong population. For each age group i in our study data (i = 1,2,3,4), which corresponds to age groups (2-18, 19-44,45-64,65+ ), we define Ai=Age group specific proportion of four fold rise in our study data x sample size of this study x Proportion of population in this age group
  2. Then, number of infections can be calculated with the following formula
  3. \( Hong\ Kong\ population\ X\frac{\sum_{i=1}^4\mathrm{Ai}}{our\ study\ sample\ size} \)
  4. bEstimated excess rate of adverse outcomes. Point estimates calculated as in [26]
  5. cBinomial confidence bounds reflecting uncertainty in the number of infections, rather than the number of adverse outcomes. See Discussion