Dr. Kavita Patel told CNBC on Tuesday that she believes the Food and Drug Administration’s recommendation that states stop using Johnson & Johnson’s single-shot Covid vaccine is likely to have a lasting impact on the country’s efforts to fight the pandemic.

“This is a devastating blow to this J&J vaccination effort in the US,” said Patel, a family doctor in Washington, DC, in an interview on Squawk Box. She also worked on health initiatives in the Obama administration while serving as director of policy for the Bureau of Interstate Affairs and Public Engagement.

Patel said the supply of Pfizer and Moderna’s two-shot vaccines will not be able to quickly meet the demand caused by the J&J hiatus. This will delay US vaccination efforts, she added.

The FDA recommendation, released Tuesday, came after six people in the US experienced rare and severe blood clotting problems after receiving the J&J vaccine.

In a tweet, the US regulator said its actions were taken “out of caution”.

All six cases occurred in women between the ages of 18 and 48, with symptoms developing six to 13 days after receiving the shot.

So far, J&J has said that there is “no clear causal link” between these rare events and the vaccine. The US drug giant also said it was working with regulators.

While she anticipates that Moderna and Pfizer will at some point be able to “fill some of that void,” said Patel, “it will be some time” before these other vaccine manufacturers have additional doses available in the US

A particular challenge in discontinuing the administration of J & J’s vaccine is that it only requires a single shot, while Moderna and Pfizer’s mRNA vaccines require two doses for complete protection of immunity.

“We just can’t replace it for the next week or three,” said Patel, a medical assistant for NBC News and a non-resident of the Brookings Institution. “This will delay our vaccination efforts.”

To compensate for this, the US could consider reducing second-dose administration to recipients of Moderna and Pfizer vaccines, Patel suggested.

The Chief Medical Officer of the White House, Dr. Anthony Fauci, has spoken out against requests earlier in the pandemic.

The second dose of Moderna is supposed to be given four weeks after the first, while Pfizer is three weeks apart.

“You will hear a renewal for calls to delay that second shot so we can get that many first shots in the arms. It’s not an unreasonable thing to think about now,” said Patel.

“If we postpone the second dose of Moderna or Pfizer for a week or two, it might actually help us fill some of that void faster,” she added.