The European Medicines Agency (EMA) has said it is recommending that people with a weak immune system receive a third dose of either the Pfizer or Moderna Covid-19 shot 28 days after the second dose is administered.

In a statement on Monday, the EMA said that a third dose would “increase the ability to produce antibodies against the virus that causes Covid-19 in organ transplant patients with weakened immune systems.”

While the agency said there’s no direct evidence indicating that this antibody production ability protected this cohort against Covid-19, it said it is “expected that the extra dose would increase protection at least in some patients.”

They contended that this measure – administering an additional Pfizer or Moderna shot just 28 days after the 2nd dose – was intended for people with “severely weakened immune systems.” The agency also said it would continue to monitor the effectiveness of the move as more data becomes available. 

The regulator also gave its support for administering Pfizer booster shots to adults with normal immune systems, at least six months after they received an initial second dose, citing real-world data. Many countries, including the UK, the US and some EU states, are already administering booster jabs.

The statement adds that the EMA will continue to observe the effectiveness of the two vaccines in general, and noted the “risk of inflammatory heart conditions or other very rare side effects after a booster is not known.” Myocarditis, the inflammation of the heart muscle, was previously noted by the agency as a potential rare side effect of the mRNA vaccines and occurred more commonly in younger adult men.

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