The latest safety alert to hit Covid-19 research efforts seems to be benign: it has been widely concluded that the death of a participant enrolled in the Brazilian trial of Astrazeneca and Oxford University’s Covid-19 vaccine, news of which emerged yesterday, was unrelated to the project, AZD1222. But future scares should be expected, simply because such huge numbers of trials of experimental agents are ongoing. Commercial companies are sponsoring almost 300 Covid-19 trials of agents that have yet to reach the market in any use, seeking around 300,000 volunteers in total, Evaluate Vantage estimates. Massive vaccine studies account for a big proportion of that enrolment number; take these out and that still leaves more than 60,000 people potentially being exposed to unapproved agents. Add in academic-led trials and those testing repurposed drugs, and the trial count quickly climbs into the thousands. At least the various halts show that safety monitoring systems are functioning. US trials of AZD1222 are due to restart this week, according to reports, while Johnson & Johnson’s vaccine candidate and Lilly’s antibody remain under review.