The adjusted estimates for cohort studies (adjusted RR=0.61, 95% CI=0.49 0.76, Phomogeneity=0.929) were somewhat different from the adjusted estimates for case control studies (adjusted RR=0.86, 95% CI=0.35 2.07, Phomogeneity=0.04) which was not statistically significant.
Therefore, the absence of adjustments in our study, when most studies adjusted by comorbidities and previous vaccination, could be a reason for the high effectiveness we observed.
Different studies adjusted for different risk factors, although broad consistency between unadjusted and adjusted models was noted.
All included studies adjusted for potential confounding covariates.
Another potential limitation is that some studies adjusted for adult height and weight, whereas other studies adjusted for BMI and this may not remove confounding effects of height on physical capability.
All studies adjusted for age and gender.
Studies adjusted for several potential confounders.
All studies adjusted their result for age.
All studies adjusted for age and sex.
All studies adjusted for baseline physical activity.
Eleven studies adjusted for smoking and seven studies adjusted for BMI.
