explain why there is a greater likelihood of rejection of male kidneys by renal patients with antiHBs, and indicate how kidneys can be better selected for transplantation”.

# **_2.  Oster’s (2005) study_**

Struck by these findings, Oster (2005) set out to test them on a population level in a large number of countries. She uses a variety of data and analytical approaches, including cross-country analyses, time-series evidence using a natural experiment based on vaccination campaigns in Alaska and Taiwan (China), and finally an estimation of the percent of the female deficit in China and India that is attributable to hepatitis B.

This is not an easy task.<sup>5</sup> Firstly, the estimates of the prevalence of hepatitis B have to be derived from scattered studies, which often represent a minuscule fraction of the country’s population. As she says: “HBV prevalence is calculated as by aggregating published studies.... Only countries with more than _2500_ people tested for HBV ( _combining all studies_ ) are used in the analysis.” (Oster 2005: footnote to Table 10, parentheses mine). Secondly, she derives an indirect estimate of the effect of HBV status on the sex ratio at birth, since the direct estimates from the microstudies were based on such small numbers.

She finds that her cross-country analysis shows a clear correlation between the sex ratio at birth and the prevalence of hepatitis B. However, she notes that due to issues of data quality, these regressions omit data from Sub-Saharan Africa, where the sex ratio at birth is low and levels of HBV prevalence are high (Oster 2005:1196). Below we discuss that the issues of data quality she raises for Africa are applicable also to many other developing countries which she includes in her analysis — including India, which she analyzes extensively.

She also finds that in Alaska the vaccination campaigns clearly reduced the sex ratio at birth, but that this relationship is less clear in Taiwan (China). In fact, the Taiwanese vital statistics show that the sex ratio at birth hovered around 109 in the years 1996-2002, and rose slightly to 110 in 2003-2004.<sup>6</sup> This does not suggest that the vaccination campaign had any significant effect on the sex ratio at birth, since as she says the vaccination campaign started in 1984, so “women who are vaccinated in 1984 would be 15 by 1999”.

She then calculates the proportion of “missing women” in China and India that can be explained by the prevalence of hepatitis B, combining her estimates of “the prevalence of hepatitis B and estimates of the effect of hepatitis on the sex ratio at birth to estimate and adjust the number of missing women.” These appear to be estimates of the impact only of the mother’s (not the father’s) HBV status, since she states that the prevalence data pertain to pregnant women only (Oster 2005: footnote to Table 10).

Using these estimates of HBV prevalence and impact on the sex ratio at birth, she estimates that up to 75 percent of the “missing women” in China in the early 1990s is attributable to parental infection with hepatitis B. Her conclusions are striking, and have received much attention.<sup>7</sup> Given their strong implications for policy, they warrant deeper exploration with more robust data.

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