The per contact transmission probability for the susceptible elderly (>64) infectees (see next to the last row in Table 2 ) is estimated to be 0.155, more than threefold of any other age group (with the young children of 0-2 being the next highest), which may be due to the common need for very close contact while the elderly (or younger children of age <3) are being cared for, typically by individuals from adults of ages 22-64 in age group 6, even though the frequency of contact might be less than that of with other age groups. This further highlights the high vulnerability of the elderly (or the younger children) to exposure from other age groups, and demonstrates the need for morbidity-based strategy to prevent the elderly (or younger children) influenza Figure 5 Simulation with the same model parameters as in Fig. 3 , except υ 1 (t) = υ 7 (t) = 0. The black dots are real data, the red curves are model fits in Fig. 3 , and the blue curves denote simulation without vaccination. mortality. On the other hand, the estimated per contact transmission probabilities for the younger (<3) and the elderly (>64) infectors are also both higher than those of the other groups (see the last row in Table 2 ), but not significantly so except when compared with those between ages 8-14. The higher transmission probabilities of young children and elderly infectives could reflect, again, the fact that contacts with these individuals are usually of a more intimate nature, although these probabilities are not as drastically different as their vulnerability to be infected. The less likelihood of schoolchildren of 8-14 to infect others might also be attributable to signs of their less intimate contacts with others as they grow into adolescence.