American health officials said Friday that the United States is re-starting the use of the Johnson & Johnson coronavirus vaccine.

The decision came after a committee of health advisors told the U.S. Centers for Disease Control and Prevention, or CDC, that the vaccine's benefits outweigh the serious but rare risk of blood system blockages, or blood clots. They also recommended that the shots come with a warning for that risk.

The CDC had called for a "pause" in the use of the Johnson & Johnson, or J & J, vaccine, on April 13. That move came after six people had developed rare blood clots within three weeks after vaccination. Health officials said Friday they found nine more cases, bringing the total to 15. All of them were women, most of them under 50.

The U.S. Food and Drug Administration had approved the J & J vaccine for emergency use in late February. So far, about eight million shots have been given, most with few or no side effects.

Health officials said anyone who has pain in their head, legs, in the middle part of their body or trouble breathing after receiving the J & J shot should contact their doctor. They also warned that a usual treatment for blood clots, the drug heparin, might be dangerous. They advised that healthcare workers should use different treatments for side effects from the vaccine.

On Tuesday, the European Medicines Agency, or EMA, also warned of a "possible link" to "very rare cases of unusual blood clots." The European health agency permitted the use of the vaccine with a warning about the rare blood clots.

Shortly after the EMA's decision, Johnson & Johnson announced that it would start sending the vaccines to European countries with added guidance for healthcare workers and patients.

Similar to AstraZeneca cases

The health agencies also noted the similarity of reports of blood clots among people receiving the J & J vaccine and the AstraZeneca coronavirus vaccine.

The AstraZeneca and J & J shots are not exact copies. But they are made with the same viral vector technology. Both use modified, harmless cold virus material as a vector, or carrier. The shot gives instructions to the human body to develop proteins to fight the coronavirus.

Viral vector vaccines can be produced quickly and in large numbers. They have been in use since the 1970s to develop vaccines against Ebola, Zika and influenza. They do not require extreme cold storage, making them easier to use in areas with limited resources.

The shots from Pfizer-BioNTech and Moderna use messenger RNA, or mRNA, technology. They are more costly to make and require extreme cold storage.

The AstraZeneca vaccine has been approved for emergency use in over 100 countries including Britain and the European Union, but not in the U.S. Two other vaccines, Sputnik V from Russia's Gamaleya Research Institute and China's shot from CanSino Biologics use viral vector technology. They are also widely available around the world.

The reports of blood clots were a second incident of bad news for Johnson & Johnson. In late March, millions of shots had to be thrown out after a manufacturing mistake was found at a factory in Baltimore, Maryland. Johnson & Johnson then took over the production in hopes of meeting its commitment to provide the United States with about 100 million shots by the end of May.

In the U.S. more than 200 million people have received at least one vaccine injection. The country has enough supplies of vaccines from Pfizer and Moderna for the rest of the population.

However, an American decision on the J & J and AstraZeneca vaccines could affect world decisions about their use. And there are increasing calls for the U.S. to share its surplus vaccine supply with the rest of the world.

I'm Mario Ritter, Jr.

Hai Do wrote this story for Learning English. Mario Ritter, Jr. was the editor.

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benefit –n. a good or helpful result or effect

pause –n. a temporary stop : a period of time in which something is stopped before it is started again

blood clot –n. a thick and sticky clump of dried blood that stops blood from flowing through a blood vessel

storage –n. space where you put things when they are not being used