This article compares the current debate over the use of placebos in developing country clinical trials of second generation Covid-19 vaccines with the debates over previous paradigmatic cases raising similar issues. Compared to the earlier zidovudine and Surfaxin trials, Covid-19 vaccine trials are likely to confer lower risk to placebo groups and to offer a greater number and variety of alternative study designs. However, turning to the developing world to conduct studies that would be unacceptable in developed countries, simply on the ground that Covid-19 vaccines are generally unavailable in developing countries, is not ethically justifiable. This is so whether the justification is rooted in total absence of vaccine in a given country or in developing country vaccine prioritisation practices, because at root both derive from economic, not scientific conditions. However, the advent of variants that may create genuine uncertainty as to comparator vaccine effectiveness could justify a placebo control, depending on vaccine characteristics, variant prevalence, the degree of variant resistance, and the acceptability of immune-bridging studies. These factors must be considered together in the necessary case-by-case assessment of the ethical justification for any proposed trial.