Using NonTargeted HPV Infections in Studies with Risk Compensation

TL;DR

This paper examines the use of non-targeted HPV infections to reduce confounding bias in observational vaccine efficacy studies, clarifying the causal interpretation and public health implications of such estimates.

Contribution

It extends previous methods by considering unmeasured behavioral confounders and mediators, providing a clearer understanding of what the estimates represent.

Findings

Using non-targeted HPV infections can remove confounding bias.

The estimated effect can be interpreted as the direct immunological effect.

Public health interpretation may be misleading under risk compensation.

Abstract

Studies of HPV vaccine efficacy usually record infections with vaccine targeted and nontargeted strains. Contrary to blinded randomized controlled trials, confounding bias can be a threat and risk compensation may occur in observational studies. Etievant et al. (Biometrics, 2023) proposed to use cervical infections with nontargeted HPV strains to reduce or remove confounding bias of estimates of vaccine efficacy on targeted strains. However, they assumed that vaccinated women could not change their behavior after vaccination. We consider a more plausible setting where unmeasured sexual behavior acts as both a confounder and a mediator, and investigate if the quantity estimated in practice with their method has a clear causal meaning. We demonstrate that using nontargeted HPV infections can remove both confounding bias and the portion of the vaccine effect on the targeted HPV strains that is mediated through the change of behavior. In that case, the estimated quantity has a clear causal interpretation as it represents the direct immunological effect of the vaccine. However, it could be considered misleading from a public health perspective, as in the presence of risk compensation it would suggest higher protection than what women effectively experience. An unblinded randomized controlled trial would allow estimation of the total causal effect of the vaccine, and infections with nontargeted HPV strains could then be used to isolate the indirect behavioral effect of the vaccine.