US regulators on 12 August said transplant recipients and others with weakened immune systems can get an extra dose of the COVID-19 vaccine to better protect them as the delta variant continues to surge.

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The announcement by the Food and Drug Administration applies to millions of Americans who are especially vulnerable because of organ transplants, certain cancers or other disorders. Several other countries, including France and Israel, have similar recommendations. It's harder for vaccines to rev up an immune system suppressed by certain medications or diseases, so those patients don't always get the same protection as otherwise healthy people and small studies suggest for at least some, an extra dose may be the solution.

This action is about ensuring our most vulnerable ... are better protected against COVID-19, Dr Rochelle Walensky, director of the Centers for Disease Control and Prevention, said ahead of the FDA's announcement. Importantly, the decision only applies to this high-risk group, about 3 per cent of US adults. It's not an opening for booster doses for the general population.

Instead, health authorities consider the extra dose part of the initial COVID-19 vaccine prescription for the immune-compromised. For example, France since April has encouraged that such patients get the third dose four weeks after their regular second shot. Separately, US health officials are continuing to closely monitor if and when average people's immunity wanes enough to require boosters for everyone but for now, the vaccines continue to offer robust protection for the general population.