BRUSSELS—The European Commission on Wednesday will extend EU powers to potentially block COVID-19 vaccine exports to Britain and other areas with much higher vaccination rates, and to cover instances of companies backloading contracted supplies, EU officials said. The regulation is aimed at making vaccine trade reciprocal and proportional so that other vaccine-making countries sell to Europe and the EU does not export much more than it imports, one EU official said. With no numerical targets, the change is unlikely to trigger mass export bans of EU-made vaccines, the official with insight into the announcement said on Wednesday. “I just really, really don’t see that happening because we have our international obligations and we want to keep supply chains going and the global system moving and flowing,” the official said. The regulation will be the basis for the EU’s 27 governments to decide whether to block vaccine exports or not. “In …