Aetiological role of common respiratory viruses in acute lower respiratory infections in children under five years: A systematic review and meta–analysis

Shi, Ting; McLean, Kenneth; Campbell, Harry; Nair, Harish

Several methodological issues could affect our results: case ascertainment, case definition, clinical specimen and confounding. Twenty-two of the 23 studies used passive hospital-based case ascertainment. Several previous studies have shown that children in developing countries, particularly those residing in rural areas, have in general, limited access to healthcare [39] , and health care seeking behaviour is often delayed or absent [40] [41] [42] . This potentially introduced a selection bias. Similarly, only three studies used community based controls [15, 19, 25] . Hospital ascertained controls may not reflect the general population, and may have other health conditions potentially affecting their viral carriage, especially those with URTI. The ideal control group for these studies would be a random sample of an age and sex matched child population from the same area of residence studied at the same time. Studies, however, recruited controls who were either selected as healthy (asymptomatic) and so biased in favour of those not exposed to the respiratory virus (yielding a falsely high OR) or those who were selected to have respiratory symptoms and so biased in favour of those who had been exposed to the respiratory virus (yielding a falsely low OR). Consistent with this interpretation, we found ( Table 2 ) odds ratios (of ALRI given viral identification) to be consistently greater where the control group were "healthy" and asymptomatic rather than symptomatic (URTI). We consider that the value of the OR based on a population-based control group as described above would lie between these two values.