Pfizer or Moderna vaccines instead of the Johnson & Johnson shot that can cause rare but serious blood clots, U.S. health officials said on 16 December.

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The strange clotting problem has caused nine confirmed deaths after J&J vaccinations while the Pfizer and Moderna vaccines don't come with that risk and also appear more effective, said advisers to the Centers for Disease Control and Prevention. The panel recommended the unusual move of giving preference to the Pfizer and Moderna vaccines, and late the CDC's director, Dr. Rochelle Walensky, accepted the panel's advice.

Until now the U.S. has treated all three COVID-19 vaccines available to Americans as an equal choice since large studies found they all offered strong protection and early supplies were limited. J&J's vaccine initially was welcomed as a single-dose option that could be especially important for hard-to-reach groups like homeless people who might not get the needed second dose of the Pfizer or Moderna options.

But the CDC's advisers said during a meeting that it was time to recognise a lot has changed since vaccines began rolling out a year ago. More than 200 million Americans are considered fully vaccinated, including about 16 million who got the J&J shot.

New data from unprecedented safety tracking of all those vaccinations persuaded the panel that while the blood clots linked to J&J's vaccine remain very rare, they're still occurring and not just in younger women as originally thought. In a unanimous vote, the advisers decided the safer Pfizer and Moderna vaccines are preferred. But they said the shot made by J&J's Janssen division still should be available if someone really wants it or has a severe allergy to the other options.

I would not recommend the Janssen vaccine to my family members but some patients may and should be able to choose that shot, said CDC adviser Dr. Beth Bell of the University of Washington. The clotting problems first came up last spring, with the J&J shot in the U.S. and with a similar vaccine made by AstraZeneca that is used in other countries.

Eventually, U.S. regulators decided the benefits of J&J's one-and-done vaccine outweighed what was considered a very rare risk as long as recipients were warned. European regulators likewise continued to recommend AstraZeneca's two-dose vaccine although, because early reports were mostly in younger women, some countries issued age restrictions.

COVID-19 causes deadly blood clots, too. But the vaccine-linked kind is different, believed to form because of a rogue immune reaction to the J&J and AstraZeneca vaccines because of how they're made.

It forms in unusual places, such as veins that drain blood from the brain, and in patients who also develop abnormally low levels of the platelets that form clots. Symptoms of the unusual clots, dubbed thrombosis with thrombocytopenia syndrome, include severe headaches a week or two after the J&J vaccination not right away as well as abdominal pain and nausea.

While it's still very rare, the Food and Drug Administration told health care providers this week that more cases have occurred after J&J vaccinations since the spring. They occur mostly in women ages 30 to 49 -- about once for every 100,000 doses administered, the FDA said.

In comparing the pros and cons of all the vaccines, the panellists agreed that side effects from the Pfizer and Moderna vaccines weren't as serious and that supplies now are plentiful. Nor is J&J still considered a one-and-done vaccine, several advisers noted. The single-dose option didn't prove quite as protective as two doses of the Pfizer and Moderna vaccines. Plus, with extra-contagious virus mutants now spreading, booster doses now are recommended.

For J&J recipients, a booster is recommended at least two months after vaccination. U.S. health officials had previously agreed mixing vaccines for booster shots.

Several countries, including Canada, already have policies that give preference to the Pfizer and Moderna vaccines. But J&J told the committee its vaccine still offers strong protection and is a critical option especially in parts of the world without plentiful vaccine supplies or for people who don't want a two-dose shot. While blood clots are rare, unfortunately, cases of COVID-19 are not, J&J's Dr. Penny Heaton said.

The U.S. is fortunate in its vaccine availability and Thursday's action shouldn't discourage use of J&J's vaccine in places around the world where it's needed, said CDC adviser Dr. Matthew Daley of Kaiser Permanente Colorado. The FDA also warned this week that another dose of the J&J vaccine shouldn't be given to anyone who developed a clot following either a J&J or AstraZeneca shot.

The committee also heard some of the first data on reported side effects of Pfizer vaccinations in younger children. Early last month, the CDC recommended a two-dose series for that age group, and more than 7 million doses have been given so far. But few problems have been reported. Of the 80 reported cases of serious side effects, about 10 involved a form of inflammation that has been seen in male teens and young adults.