Medically reviewed in April 2021
Updated on April 26, 2021
Health officials have lifted their pause of the Johnson & Johnson COVID-19 vaccine, which has put the one-dose shot back into circulation in the United States. They caution, however, that women younger than 50-years old should be aware of the risk of a rare blood clotting syndrome associated with the vaccine.
The Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA) made the decision following the recommendation of the CDC's Advisory Committee on Immunization Practices (ACIP), an independent panel of experts. The ACIP met on Friday, April 23 to review more evidence about the rare blood clots reported in a handful of people who received the J&J vaccine.
The ACIP decided that the rollout of the J&J shot should restart, noting the vaccine should come with a new warning. The panel concluded that the benefits of the J&J COVID vaccine outweigh this risk.
Why the J&J vaccine rollout was paused
The Centers for Disease Control’s Advisory Committee on Immunization Practices (ACIP) met on Friday, April 23 to review more evidence about the rare blood clots reported in a handful of vaccinated people.
The J&J vaccine rollout was halted at federal sites on April 13, and health officials urged states to do the same. At the time, six women between the ages of 18 and 48 had developed a very rare condition, known as cerebral venous sinus thrombosis (CVST), in combination with low levels of blood platelets (thrombocytopenia), 6 to 13 days after they received the J&J shot.
Health officials noted the move was essential to ensuring that healthcare providers across the country are aware of the potential for these adverse events, according to Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, and Dr. Anne Schuchat, Principal Deputy Director of the CDC, in an April 13 joint news release.
The pause gave them time to plan for the diagnosis and management of this rare cluster of symptoms.
Scientists are now calling the blood clotting syndrome—characterized by rare blood clots in the brain (and possibly in other parts of the body) combined with a low platelet count—thrombosis with thrombocytopenia syndrome, or TTS.
Since the vaccine was halted, the ACIP was presented with a few more cases. Overall, 15 people developed TTS after being vaccinated. Of these people, five are now home, seven remain hospitalized and three died.
Since these initial cases of TTS were reported, however, health officials have learned more about how to effectively manage TTS.
“Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given,” advised Dr. Marks and Dr. Schuchat.
What is TTS?
TTS is characterized by CVST or clots in other parts of the body in combination with low levels of blood platelets.
CVST occurs when a blood clot forms in the brain, which prevents blood from draining. This can cause blood cells to break and leak into the brain.
It’s very rare. It affects about five out of one million people each year.
Prompt medical attention is key for recovery from CVST. Treatment, which should begin right away, may include fluids, antibiotics if infection is present and medicine to control seizures or stop the blood from clotting.
CVST is usually manageable if detected and treated early. But the condition may be missed or diagnosed late because its symptoms overlap with other conditions. Responding quickly to the following warning signs improves the likelihood of recovery:
- Headache
- Blurred vision
- Fainting or loss of consciousness
- Loss of control over movement in part of the body
- Seizures
- Severe abdominal or leg pain
- Shortness of breath
Additional warning signs of low platelet count include:
- New easy or unexplained bruising
- Tiny red spots on the skin (petechia)
Who is at higher risk for TTS?
Among women, those younger than 50-years old are at higher risk for TTS, including CVST, following vaccination with the J&J shot. Women between 30 and 39, in particular, are more likely to be affected.
Other possible risk factors for the condition include obesity, use of oral contraceptives, hypothyroidism, and high blood pressure.
Keep in mind, cases of TTS were reported within two weeks of vaccination. Risk for the condition decreases over time. So, anyone who received the vaccine more than three weeks ago is at much lower risk.
COVID-19 more likely to cause blood clots
News about rare blood clots associated with the J&J vaccine is worrisome. But it’s important to keep this in perspective. The odds for TTS are seven out of one million for women younger than 50-years old and fewer than one per one million doses for men and women older than 50.
So, the risk for blood clots is actually much greater among those with COVID-19.
A virus like SARS-CoV-2, which causes COVID-19, can lead to inflammation that triggers an overwhelming release of proteins called cytokines. Normally, cytokines help coordinate the body’s response to infection. But in some cases, this response can range out of control, doing more harm than good. This so-called cytokine storm can damage the blood vessels and organs and lead to blood clots.
Why were blood clots not detected sooner?
Phase 3 trials of the J&J vaccine were conducted in eight countries across three continents and included about 40,000 people of different ages, races and ethnicities.
Overall, the vaccine was 66 percent effective at preventing moderate to severe disease 28 days after immunization. The most commonly reported side effects included pain at the injection site, headache and muscle aches.
Some adverse events may not be detected until millions of people receive the vaccine.
Since it was authorized for use in the U.S. by the FDA and approved by the CDC on February 28, more than 8 million doses of the J&J vaccine have been doled out. Of these people, there have so far been 15 reported cases of TTS, making this an extremely rare event.
Safety monitoring is working
COVID-19 vaccines have undergone and will continue to undergo the most intensive safety monitoring in U.S. history.
The halted rollout of the J&J vaccine and the investigations that followed are a reassuring sign that vaccine safety reporting systems are working.
The U.S. Food and Drug Administration has existing systems to continue monitoring for efficacy and safety after a vaccine is approved. This can include requiring companies to conduct long-term safety and efficacy studies, called Phase 4 studies. The FDA also continues to inspect manufacturing facilities and test new batches of vaccines.
Several monitoring systems are also in place. These include the Vaccine Adverse Event Reporting System (VAERS), a voluntary reporting system run by the CDC and FDA, the FDA’s Sentinel Initiative, and the CDC’s Vaccine Safety Datalink (VSD).
The CDC’s V-safe After Vaccination Health Checker and pregnancy registry are other surveillance programs that are tracking the COVID vaccines’ effects among people who voluntarily register. The agency also runs Rapid Cycle Analysis (RCA), which monitors safety data in real time and can alert health officials to problems as they happen so that the public can be notified quickly.
Why are the J&J and AstraZeneca vaccines linked to blood clots?
The AstraZeneca (AZ) vaccine is not authorized for use in the United States. But both the AZ and J&J shots are viral vector vaccines.
They use a modified version of another type of virus as a vector (or mode of delivery) for genetic material that codes for a piece of the coronavirus in order to trigger a protective immune response.
The viral vector used by the J&J vaccine is a common cold virus called adenovirus 26. AstraZeneca uses a weakened version of a chimpanzee adenovirus.
Scientists speculate that these vaccines could be triggering an immune response against platelet factor 4, a protein involved in blood clotting. This results in platelet destruction and release of clot-promoting material.
AstraZeneca reported the most recent results from its large U.S. clinical trial on March 25. The company says that its two-dose vaccine, called AZD1222, was shown to be 76 percent effective at preventing symptomatic COVID-19 and 100 percent effective at preventing severe infection and hospitalization.
The vaccine was also shown to be safe and well-tolerated among trial participants. No serious safety concerns were reported. This was a particularly important finding as several countries recently halted use of the vaccine following reports of blood clots in some vaccinated people.
In the U.S., the Data Safety Monitoring Board conducted a specific review of clotting issues, including CVST combined with low platelet count. Among more than 21,000 people, the monitoring board found no increased risk for clotting events.
But again, very rare side effects are not usually identified until a drug has been given to millions of people.
What about the Pfizer and Moderna shots?
In the U.S., the vast majority of the vaccine supply comes from Pfizer-BioNTech and Moderna.
As of April 23, more than 213 million doses of these mRNA vaccines have been administered. There have been no serious safety concerns associated with either of these vaccines.
