COLUMBUS, Ohio (WCMH) — The U.S. Food and Drug Administration is shifting its guidance on the use of the Johnson & Johnson COVID-19 vaccine going forward.

“This is what we know — the J&J vaccine is associated with a very serious type of brain blood clot, and there’s roughly 3.2 cases per one million doses,” Dr. Joseph Gastaldo, an infectious disease physician at OhioHealth, said.

Those blood clots have led to nine deaths so far in the United States, and Gastaldo said where it’s available, patients should opt for Moderna or Pfizer, both of which are mRNA vaccines.

“It should not be used unless somebody for whatever reason cannot take the mRNA vaccine, or the patient really doesn’t want to take an mRNA vaccine,” Gastaldo said. “The biggest reason why somebody should not get an mRNA vaccine is if they have a severe allergic reaction to either the first dose or a known component of the mRNA vaccine.”

Because the blood clots are extremely rare, Gastaldo said Ohioans who already received the J&J vaccine shouldn’t sound the alarm.

“If you’ve received the J&J vaccine many months ago, you have nothing to worry about as far as getting that clot,” Gastaldo said.

But, for new recipients, experts warn the symptoms of a blood clot tend to form within one to two weeks.

“Headache, nausea, visual changes, loss of consciousness,” Gastaldo said. “The point is, if you have any concerns about that very rare type of blood clot — seek medical care.”

Of the 600 million doses of COVID-19 vaccines given in the U.S., only 19 million have been the Johnson & Johnson vaccine.

Though some protection against the virus is better than none, experts are now echoing the FDA’s latest guidance.

“The single dose J&J vaccine really does not perform as well as an mRNA vaccine,” Gastaldo said. “Both mRNA vaccines are fully FDA-approved for the first two doses, and they really have a proven safety and efficacy record.”

Gastaldo said that if you did receive the Johnson & Johnson vaccine knowing it doesn’t perform as well, it’s important to be up to date with your vaccines and get the appropriate recommended boosters.