How to Think About Johnson & Johnson Vaccine and Rare Side Effects

Federal drug safety regulators have recommended pausing the use of Johnson & Johnson’s COVID-19 vaccine after six women experienced blood clots after vaccination. Matthew Hatcher/Getty Images

Federal drug safety regulators have rec…

Federal drug safety regulators have recommended pausing the use of Johnson & Johnson’s COVID-19 vaccine after six women experienced blood clots after vaccination. Matthew Hatcher/Getty Images
  • Federal drug safety regulators have recommended pausing the use of Johnson & Johnson’s COVID-19 vaccine after six women experienced blood clots after vaccination.
  • The clots are rare but serious, and may be caused by an unusual immune reaction that triggers clotting.
  • Some health experts say temporarily halting the use of the Johnson & Johnson shot could slow the vaccine rollout and fuel vaccine hesitancy. 

Federal drug safety regulators have recommended pausing the use of Johnson & Johnson’s COVID-19 vaccine after six women experienced blood clots after vaccination. 

Of the six women who experienced clots, one died and another is hospitalized. 

Experts are still investigating whether the vaccine was the cause of these events.

The Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) recommended halting the use of the vaccine to alert healthcare professionals about the clots and ensure they’re prepared to diagnose and treat them. 

The clots appear to be similar to the unusual, rare clots reported in a handful of people who received the AstraZeneca COVID-19 vaccine. 

Many health experts say temporarily halting the use of the Johnson & Johnson shot — a vaccine that’s highly effective at preventing hospitalization and death from COVID-19 — could slow the vaccine rollout and fuel vaccine hesitancy. 

Studies suggest the shots could be triggering an immune response that causes clotting, but more research is needed to better understand the potential link. 

What’s going on with the clots?

It’s unclear what exactly is causing the clots, but recent research suggests the shots may trigger an immune response that facilitates blood clotting. 

Two recent studies published in The New England Journal of Medicine explored the possibility that a small number of people vaccinated with the AstraZeneca shot who experienced clots may have developed cerebral venous sinus thrombosis (CVST) in combination with low levels of blood platelets, called thrombotic thrombocytopenia. 

CVST is known to occur in a small percentage of people who receive heparin, a type of blood thinner.

In these instances, the people experienced clots in unusual locations, like veins in the brain and abdomen. They’re developing low platelet counts and have formed high levels of antibodies against platelets, explains Dr. Robert Bona, a Yale Medicine hematologist and professor of medicine at Yale School of Medicine.

Both the AstraZeneca and Johnson & Johnson COVID-19 vaccines are adenovirus vaccines. In this type of vaccine, a modified version of a virus (also called a vector) is used to deliver instructions to the immune system to help prevent infections.

According to Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security and an infectious disease expert, the similarity of the rare clots reported after both the Johnson & Johnson and AstraZeneca shots may suggest that these adenovirus vectors may be linked to risk of developing blood clots. 

“This vector may, in certain individuals, generate antibodies that interfere with platelet function and produce a condition similar to heparin-induced thrombocytopenia,” Adalja said. This condition “is diagnosable and treatable.”

None of the people who developed clots after vaccination had been recently given heparin.

“It’s an unusual situation where people are developing low platelets, and sometimes very low platelets, and blood clotting at unusual sites in association with high levels of antibodies to this protein called platelet factor 4 in the absence of any prior exposure to heparin,” Bona said.

There are other clinical reports of thrombocytopenia occurring in people who hadn’t been exposed to heparin but developed an autoimmune response. 

These types of clots are rare

When hundreds of millions of people are getting vaccinated against COVID-19, we’re going to see some routine blood clots occur around the time of vaccination just as a coincidence, according to Bona.

“There’s going to be some background blood clotting, because blood clots occur in 1 to 2 per 1,000 people per year,” Bona said.

In general, CVST is rare. It typically occurs in about 5 out of 1 million people annually. 

The potentially vaccine-induced clots appear to be even rarer. Of the nearly 7 million people who received the Johnson & Johnson vaccine, six developed clots — that prevalence equates to 0.000088 percent. 

The risk of experiencing clots after vaccination “is pretty small, but still obviously to the people who are getting this it’s very significant,” Bona said. 

In the United States, the coronavirus, in comparison, has killed more than 563,000 people. More than 31.3 million have contracted it.

Globally, there are more than 137.8 million confirmed cases of COVID-19. Nearly 3 million people have died.

“This is less than 1 and 1 million and certainly less than the risk of blood clots from getting COVID itself,” Adalja said of the clots reported after the Johnson & Johnson vaccine. 

The clots can be diagnosed and treated

The tip-off that someone may be developing clots would be lab test results that find low platelets or certain kinds of antibodies.

For the heparin-induced and autoimmune-triggered thrombocytopenia, where people also develop these platelet factor 4 antibodies, one effective treatment is immunoglobulin delivered intravenously.

Certain blood thinners also appear to be more effective than others, said Bona. 

“That’s what’s being recommended right now as the approach to the treatment of the individuals” who experience clots after vaccination, Bona said. 

With heparin-induced thrombocytopenia, these medications are highly effective. 

Do the benefits of the shot outweigh the risks? 

Many health experts, including Adalja, believe the benefits of the AstraZeneca and Johnson & Johnson COVID-19 vaccines far outweigh the rare risks. 

The Johnson & Johnson shot is 100 percent effective at preventing hospitalization and death. It’s 85 percent effective at preventing severe disease and 66 percent effective at preventing symptomatic infection (which includes mild infections). 

Adalja expects regulators to ultimately say there's a risk-benefit ratio that strongly favors the vaccine, leading them to lift the federal pause on the Johnson & Johnson shot. 

The AstraZeneca shot was temporarily paused in Europe, but officials eventually concluded the benefits of vaccinating people against COVID-19 outweighed the small risk of clots. 

European health officials also recommended that people under age 30 receive one of the other vaccines, like Pfizer-BioNTech or Moderna. 

Many experts believe that temporarily pausing the use of these vaccines could ultimately slow the vaccine rollout and fuel vaccine hesitancy. 

“This action will likely add to vaccine hesitancy and will do irreparable harm to the vaccine rollout and will bleed into the other vaccines as well,” Adalja said. 

The bottom line

Federal drug safety regulators have recommended pausing the use of Johnson & Johnson’s COVID-19 vaccine after six women experienced blood clots after vaccination.

The clots are rare but serious, and may be caused by an unusual immune reaction that triggers clotting.

Some health experts say temporarily halting the use of the Johnson & Johnson shot — a vaccine that’s highly effective at preventing hospitalization and death from COVID-19 — could slow the vaccine rollout and fuel vaccine hesitancy.