Table 4.

The effect of exposure on the rate of pneumococcal acquisition in day care attendees.

Exposure stratum	Person-months* in children at risk of acquisition	Number of acquisitions	Rate of acquisition per month per 100 children (90% confidence interval)
None	7732	35	0.45 (0.34 – 0.60)
DCC only	1074	27	2.51 (1.83 – 3.45)
Family and	64	8	12.50 (7.00–22.32)
DCC
Family only	22	1	4.55 (0.88–23.43)
Total	8892	71	0.80 (0.66 – 0.97)

Acquisition rates were determined in the following exposure strata: no observed carriage, carriage in the day care centre (DCC) only, carriage in both and carriage in the family only. The acquisition rates were calculated from the number of acquisitions for children at risk, i.e., not carrying the target type at the previous visit. The same acquisition rate was assumed for all serotypes.

* person-months were calculated from the number of children at risk at the start of the one-month observation interval, each contributing one month of person-time