GACVS Statement on Safety Signals with Johnson&Johnson/Janssen COVID-19 vaccine


The COVID-19 subcommittee of the WHO Global Advisory Committee on Vaccine Safety (GACVS) met virtually on 11 May 2021 to review available data on thromboembolic events (blood clots) and thrombocytopenia (low platelet levels) following administration of the Johnson & Johnson adenoviral vector vaccine. The data were from stimulated passive surveillance and a small active surveillance cohort of J&J COVID-19 vaccine recipients in the United States, where more than eight million doses had been administered as of 7 May 2021. 

Current evidence suggests a plausible causal association between the Johnson & Johnson COVID-19 vaccine and thrombosis with thrombocytopenia syndrome (TTS), for which the clinical characteristics appear to be similar to those seen with the AstraZeneca COVID-19 vaccine.  The exact mechanism by which this rare condition occurs is not yet fully understood, and to date the only possible risk factors identified are age and sex (with more cases reported in women than in men). TTS does not appear to be associated with mRNA COVID-19 vaccines.

After a review of the available data, the GACVS COVID-19 subcommittee issued the following conclusions and recommendations:

  • The benefits of the Johnson & Johnson COVID-19 vaccine continue to outweigh the risks of TTS. As it is the only single-dose COVID-19 vaccine approved for use to date, this vaccine could be important for hard-to-reach populations.
  • As of 7 May 2021, the U.S. Food and Drug Administration (FDA) and the Centers for Disease Control and prevention (CDC) had reviewed 28 reports of TTS out of a total of more than 8 million vaccines administered. TTS was reported in individuals ages 18 to 59 (median age 40) and occurred between 3 and 15 days (a median of 9 days) after administration of the vaccine.  
  • Although most cases of TTS have involved thrombosis in unusual locations, including cerebral venous sinuses, portal veins, splenic veins, and other rare venous and arterial thromboses, cases involving thrombosis at more common locations, such as deep vein thrombosis and pulmonary embolism, have also been identified in the United States.
  • When formulating immunization policies, the risk of TTS from use of the Johnson & Johnson COVID-19 vaccine should be weighed against the benefits. Countries assessing the risk of TTS following COVID-19 vaccination should conduct a risk-benefit analysis that takes into account local epidemiology (including incidence and mortality from COVID-19 disease), the age groups being targeted for vaccination, and the availability of alternative vaccines.
  • The appropriate information should be provided to health professionals and people who are vaccinated, to recognize the signs and symptoms of all serious adverse events following vaccination with all COVID-19 vaccines, so that people can seek and receive timely and relevant medical care and treatment. Early identification of TTS is important so that the appropriate treatment can be initiated.
  • Doctors should watch for any new, severe, and persistent headaches or other major symptoms, such as severe abdominal pain and shortness of breath, that begin four to 20 days after administration of adenovirus-based COVID-19 vaccines.
  • At a minimum, countries should encourage physicians to measure platelet levels and conduct appropriate investigation of potential thrombosis. Doctors should also be aware that although heparin is generally used to treat blood clots, administering heparin in TTS can be dangerous, and alternative treatments, such as immunoglobulins and heparin-free anticoagulants, should be considered.
  • It is recommended that countries continue to monitor the safety of all vaccines against COVID-19 and promote the reporting of suspected adverse events. In particular, any blood clots that occur after administration of any of the COVID-19 vaccines should be reported.
  • Ongoing assessment and review of cases of TTS should therefore include all vaccines that use adenoviral vector platforms.

The GACVS COVID-19 subcommittee will continue to review safety data for all COVID-19 vaccines and, when necessary, update its recommendations. The WHO COVID-19 vaccine safety surveillance manual provides guidance to countries on monitoring safety and exchanging data on adverse events associated with new COVID-19 vaccines.

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