In this April 8 file photo, the Johnson & Johnson COVID-19 vaccine is seen at a pop up vaccination site in the Staten Island borough of New York. The Centers for Disease Control and the Food and Drug Administration announced Friday that distribution and administration of the Johnson & Johnson vaccine could resume nationwide. (AP Photo/Mary Altaffer, File)

After a brief pause, the Centers for Disease Control (CDC) and the Food and Drug Administration (FDA) announced Friday that distribution and administration of the Johnson & Johnson vaccine could resume nationwide.

When several women who received the one-shot Johnson & Johnson dose developed blood cots six to 15 days afterward, the administration of the vaccine was suspended April 13 due to “an abundance of caution.”

Upon initial reports, the Advisory Committee on Immunization Practices (ACIP) launched an emergency investigation. The ACIP review panel determined that there have been seven cases per million doses among women between ages 18-49 who had rare and severe blood clots, also known as thrombocytopenia syndrome (TSS), after receiving the Johnson & Johnson vaccine.

With less than one in one million results of TSS, the ACIP concluded that the Johnson & Johnson vaccine is a safe and viable COVID-19 vaccine option, according to a San Miguel County announcement this week.

The same day, The Colorado Department of Public Health and Environment and the Colorado Joint Vaccine Task Force aligned with the findings and authorized the vaccine to return to state clinics, effective immediately.

“The measures taken by the FDA, CDC and the state not only confirm that the proper precautions are in place, but also that the health and safety of all is of greatest importance,” county public health director Grace Franklin said. “This single-dose vaccine is critical to the protection and recovery of rural communities like San Miguel County. We are excited to return to offering two options to our residents through our three vaccine distribution centers.”

Franklin had previously stated that the county would follow the state’s lead in regards to resuming the Johnson & Johnson vaccines, which the county had been receiving and administering up until the reported potential side effects were cause for pause.

In announcing that it is safe to resume the use of Johnson & Johnson vaccines, the CDC and FDA explained that the available data shows that the chance of TTS occurring in women recipients is “very low,” according to a joint news release, though both organizations “will remain vigilant in continuing to investigate this risk.”

“Safety is our top priority,” Dr. Janet Woodcock, the acting FDA commissioner, said in a news release announcing the pause's end Friday. “This pause was an example of our extensive safety monitoring working as they were designed to work — identifying even these small number of cases.”

Health care providers have been given updated information on the risks of this potential syndrome, as the FDA explained the vaccine’s known and potential benefits outweigh its known and potential risks in individuals 18 years of age and older.

“An unusual aspect of these clots is that they occur with a low platelet count. Platelets are cells in the blood that typically aid in clotting. TTS is likely caused by the Johnson & Johnson vaccination rarely causing the production of an antibody against part of a platelet called Heparin-Platelet factor 4,” explained Dr. William Petri, University of Virginia professor of medicine, according to an Associated Press report. “These antibodies induce the platelets to cause clots. TTS appears similar to a rare side effect from the blood thinner heparin called heparin-induced thrombocytopenia.”

He added, “The clots from TTS are treated with what are called directly acting anticoagulants, and not with heparin. This was part of the reason for the pause, so that word could get out to health care providers on how to properly treat this rare form of blood clots.”

Petri explained that symptoms can include severe headache, nausea, vomiting and abdominal pain followed by weakness on one side of the body, difficulty in speaking, seizures, and loss of consciousness.

“My recommendation if you are a woman under the age of 50 that you be immunized with the Pfizer or Moderna vaccines, and not the J&J vaccine. This is because this rare side effect has been observed only in young women; 13 of the 15 women who developed TTS are in this age group. For everyone else, the J&J vaccine offers the convenience of needing one and not the two doses of the other vaccines,” he said.

While TTS has typically only occurred in women under the age of 50, a San Francisco man in his 30s developed a clot in his leg within two weeks of receiving the Johnson & Johnson vaccine, according to an Associated Press report this week. The man, who has not been identified, is recuperating, according to University of California, San Francisco officials.

“To the best of our knowledge, this is the first male patient with VITT (vaccine-induced prothrombotic immune thrombocytopenia) syndrome in the U.S. following the U.S. emergency authorization of the Johnson & Johnson vaccine on Feb. 27, 2021,” the university said in a statement.

Editor’s note: Petri’s piece can be read in its entirety at theconversation.com/us/covid-19.