The FDA and CDC Pause Use of Johnson & Johnson’s COVID-19 Vaccine

By Amichai Perlman, PhD, PharmD
Medically reviewed checkmarkMedically reviewed
April 14, 2021

In a joint statement, the FDA and CDC announced yesterday that after vaccinating nearly 7 million people, the use of Johnson & Johnson’s vaccine will be paused for a few days due to six reported cases of blood clots. Here, K’s vaccination expert, Dr. Amichai Perlman, shares insight on what this means, and what you should know. 

To start, the number of clots is not a cause for alarm statistically—among 7 million people, hundreds of blood clots would normally be expected over a couple of weeks regardless of the vaccine.

However, the type of clots reported are very severe and rare, and resemble those reported in recent weeks with AstraZeneca’s (AZ) vaccine in Europe and the UK. So now a bit about AZ’s vaccine.

The AZ vaccine was given to over 20 million people in the UK, and is suspected to be linked to ~80 cases of a specific type of very rare but very serious brain blood clot, occurring primarily in people under age 50, within one to three weeks of vaccination. These clots were accompanied by a low level of blood platelets, and reports have shown them to be associated with the development of antibodies to a substance related to blood platelets.

The link is not definite. The cases are very rare (four in 1 million) and the vaccine does not appear to be linked to an increase in other types of clots. However, this type of clot is normally even more rare, the distinct clinical and laboratory features support the link with the vaccine, and when it does happen it causes significant harm to relatively young individuals.

Why Are Countries Like Ours Pausing Use?

That is why most authorities have recently recognized this as a possibly-related rare side effect of the AZ vaccine. Some countries have chosen to limit use of this vaccine in those under age 50-60, where risk of the side effect increases, and risk of COVID decreases, others have not, as this is still very rare and the benefit is expected to far outweigh risks in most people.

The FDA has chosen to act promptly in response to the six cases with the J&J vaccine, as it is similar to AZ. Both AZ and J&J vaccines use “viral vectors,” whereby a benign adenovirus is genetically modified to include DNA coding for the coronavirus spike protein (and to not replicate in the human body).

The scientists reporting on the cases with the AZ vaccine, hypothesized that in rare cases, the adenovirus vector (or its DNA) may cause the development of antibodies which interact with the platelets and lead to the rare clots.

This explanation may also account for the FDA’s observations that no such cases have been known to occur following 180 million doses of the Pfizer and Moderna vaccines. The Pfizer and Moderna vaccines do not use a viral vector, but rather are based on mRNA encapsulated in tiny soap like bubbles.

Understanding the Risk

To sum, a rare and severe type of blood clot, occurring within about one to three weeks of vaccination, has been associated with the AZ vaccine. The clot is not definitively caused by the vaccine, but there is a basis to suspect it may be.

The FDA is now investigating whether the J&J vaccine may be related to similar rare incidents. The risk is currently being evaluated, but if the risk does exist, it is likely extremely low, similar to the severe and rare risks associated with many common treatments, and orders of magnitude lower than the risks related to having COVID-19.

We will continue to keep you apprised of news and insights on COVID-19 and vaccinations as information develops. Be sure to check out our Coronavirus FAQ for information and resources about all COVID-19 vaccinations and eligibility. 

Update as of April 26, 2021

  • This past Friday, April 23d, the FDA & CDC lifted the recommended pause on the J&J COVID-19 vaccine following a thorough safety review.
  • The safety review identified a total of 15 cases of this blood clot occurring in women aged 18 to 59 within three weeks of vaccination. One case had reportedly occurred in a young man during the vaccine’s clinical trial. 
  • The FDA & CDC concluded that this severe blood clot is plausibly related to the vaccine, however it is extremely rare and that the benefits of the vaccine far outweigh the risk.
  • For additional information see:

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Amichai Perlman, PhD, PharmD

Dr. Perlman is a clinical pharmacist and pharmacoepidemiologist, with over 10 years of experience advising patients and clinicians on medication use, personalization, and safety. He has extensively published peer-reviewed research addressing medication safety.