Sampling strategies to measure the prevalence of common recurrent infections in longitudinal studies

<p>Abstract</p> <p>Background</p> <p>Measuring recurrent infections such as diarrhoea or respiratory infections in epidemiological studies is a methodological challenge. Problems in measuring the incidence of recurrent infections include the episode definition, recall error, and the logistics of close follow up. Longitudinal prevalence (LP), the proportion-of-time-ill estimated by repeated prevalence measurements, is an alternative measure to incidence of recurrent infections. In contrast to incidence which usually requires continuous sampling, LP can be measured at intervals. This study explored how many more participants are needed for infrequent sampling to achieve the same study power as frequent sampling.</p> <p>Methods</p> <p>We developed a set of four empirical simulation models representing low and high risk settings with short or long episode durations. The model was used to evaluate different sampling strategies with different assumptions on recall period and recall error.</p> <p>Results</p> <p>The model identified three major factors that influence sampling strategies: (1) the clustering of episodes in individuals; (2) the duration of episodes; (3) the positive correlation between an individual's disease incidence and episode duration. Intermittent sampling (e.g. 12 times per year) often requires only a slightly larger sample size compared to continuous sampling, especially in cluster-randomized trials. The collection of period prevalence data can lead to highly biased effect estimates if the exposure variable is associated with episode duration. To maximize study power, recall periods of 3 to 7 days may be preferable over shorter periods, even if this leads to inaccuracy in the prevalence estimates.</p> <p>Conclusion</p> <p>Choosing the optimal approach to measure recurrent infections in epidemiological studies depends on the setting, the study objectives, study design and budget constraints. Sampling at intervals can contribute to making epidemiological studies and trials more efficient, valid and cost-effective.</p

Vitamin a supplementation does not affect infants' immune responses to polio and tetanus vaccines.

It has been suggested that administering vitamin A with the measles vaccine may reduce the vaccine's immunogenicity. This trial examined the effect of supplementing vitamin A during the early months of life on infants' immune responses to tetanus and polio vaccines. Young infants (n = 1085) were enrolled and individually randomized into 1 of 4 groups in a factorial, double-blind, placebo-controlled trial. Three vitamin A supplementation strategies were investigated: 1) supplementation of breast-feeding mothers with 60 mg retinol equivalent (RE) vitamin A within 4 wk of delivery; 2) Expanded Program on Immunization (EPI)-linked supplementation of infants with 7.5 mg RE vitamin A at 6, 10, and 14 wk; and 3) combined mother and child supplementations. A 4th group in which mother and child were given placebos served as controls. Blood samples were collected from each child at 6 wk and 6 mo of age to measure antipolio antibody titer, antitetanus toxoid antibodies, and avidity of antibodies to tetanus. Of the infants randomized into the 4 arms of the study, 767 (71%) completed follow-up at 6 mo of age. Follow-up rates were similar in all 4 arms (69-72%, P = 0.8). Antibody titers were relatively high in all 4 groups at both 6 wk and 6 mo of age, with no differences among the groups. We found no evidence that vitamin A supplementation affects infants' antibody responses to tetanus toxoid or oral polio vaccine delivered at EPI contacts