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Figure 3. The relationship of invasive pneumococcal disease to
photoperiod, temperature, and precipitation in three surveillance areas,
grouped by latitude, 1996–1998. Southern surveillance areas were in Georgia
and Tennessee (red lines; 33°–35° north latitude), middle areas were in
California and Maryland (green lines; 37°–39° north latitude), and northern
areas were in Connecticut, Minnesota, and Oregon (blue lines; 41°–45°
north latitude). a: Rates of invasive pneumococcal disease; b: hours of
darkness calculated for the 15th day of each month; c: mean monthly surface
air temperature recorded at weather stations in each of the seven surveillance
sites. d: total monthly precipitation recorded at weather stations in
each of the seven surveillance sites. Pneumococcal disease correlated
directly with photoperiod (r 0.85 with a 5-week lag; p<0.0001),
indirectly with temperature (r –0.82 with a 1-week lag; p<0.0001),
and poorly with precipitation (r 0 to <0.3).
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