The Food and Drug Administration amended the emergency use authorizations for the Moderna and the Pfizer-BioNTech Covid-19 vaccines Thursday to allow some people with compromised immune systems to get a third dose.

The change in FDA regulation is specific to patients who have been unable to mount an adequate immune response against the virus, even after being fully vaccinated. The hope, experts say, is that the additional dose will provide these patients with levels of protection seen in people without weakened immune systems.

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