Augmented case-only designs for randomized clinical trials with failure time endpoints

Under suitable assumptions and by exploiting the independence between inherited genetic susceptibility and treatment assignment, the case-only design yields efficient estimates for subgroup treatment effects and gene-treatment interaction in a Cox model. However it cannot provide estimates of the genetic main effect and baseline hazards, that are necessary to compute the absolute disease risk. For two-arm, placebo-controlled trials with rare failure time endpoints, we consider augmenting the case-only design with random samples of controls from both arms, as in the classical case-cohort sampling scheme, or with a random sample of controls from the active treatment arm only. The latter design is motivated by vaccine trials for cost-effective use of resources and specimens so that host genetics and vaccine-induced immune responses can be studied simultaneously in a bigger set of participants. We show that these designs can identify all parameters in a Cox model and that the efficient case-only estimator can be incorporated in a two-step plug-in procedure. Results in simulations and a data example suggest that incorporating case-only estimators in the classical case-cohort design improves the precision of all estimated parameters; sampling controls only in the active treatment arm attains a similar level of efficiency.

NESTED CASE - CONTROL ANALYSIS OF THE RISK FACTORS FOR HIGH ALTITUDE PULMONARY OEDEMA

A nested case-control study was undertaken on a cohort of soldiers inducted into high altitude area (11000 to 16000 feet) of Western Himalayas, with the objectives of studying the incidence of high altitude pulmonary oedema (HAPO) and its association with physical exertion and certain other predetermined risk factors. The study indicated that the cumulative incidence of HAPO was 1.42 per 1000 inductions. The association with moderate/strenuous physical exertion within 24 hours of entry into high altitude was significant (Odds ratio (OR) = 3.19; 95% confidence limits (CL) = 1.23 to 8.15); however, this association was not significant for the period 24 to 48 hours or > 48 hours. Physical exertion during first 24 hours was also significantly associated with severity of disease (OR = 14.67, 95% CL = 3.61 to 64.04), but not after 24 hours. Previous history of "high altitude sickness" was also significantly associated with HAPO (OR = 2.74, 95% CL = 1.12 to 6.77). Physical exertion during first 24 hours was found to carry an attributable risk of 2.56 per 1000 inductions and an etiologic fraction of 17.8%. No significant association of HAPO was observed with age, type of inductee (fresh/reinductee), native place, alcohol consumption and smoking.