Practice Alert

Why mRNA COVID vaccines are preferred (and why patients should be reassured)

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On December 16, 2021, the Advisory Committee on Immunization Practices (ACIP) voted to preferentially recommend messenger RNA (mRNA) vaccines over the Johnson & Johnson/Janssen (J&J) COVID-19 (Ad.26.COV2.S) adenovirus vector vaccine for prevention of COVID-19.1 The mRNA vaccines include Pfizer-BioNTech COVID-19 (BNT162b2) and Moderna COVID-19 (mRNA-1273).

The reason for this preferential recommendation is a rare but serious adverse reaction—thrombosis with thrombocytopenia (TTS) —that has been associated with the J&J vaccine. As of December 8, 2021, more than 16.9 million doses of the J&J COVID-19 vaccine have been given in the United States. The CDC has identified 57 confirmed reports of people who received this vaccine and later developed TTS.2 The known incidence of TTS is thus 1 per ~ 300,000 doses, although the rate may actually be higher.2 All cases have been documented as having occurred after administration of the J&J primary single-dose vaccine; none have been documented (so far) after the booster—although the number of booster doses of the J&J COVID-19 vaccine has been small.

Women between the ages of 30 and 50 years have the highest risk for TTS, with rates of 1 per 94,000 in those ages 30-39 and 1 per 111,000 for those ages 40-49.2,3 All those with TTS have been hospitalized, and 9 have died.2,3 While this adverse reaction is rare, the seriousness of it led the ACIP to state a preference for the mRNA vaccines.

The significance of the recommendation:

  • Unless a person has a contraindication to an mRNA vaccine, they should receive 1 of these 2 vaccines for their primary series and boosters.
  • The only “Mix and Match” that should occur with boosters is to follow a J&J/Janssen COVID-19 vaccine with an mRNA booster. At this time, booster doses following a 2-dose mRNA primary series should be with an mRNA vaccine.
  • The recommendation is for adults ages 18 and older; however, the J&J/Janssen COVID-19 vaccine is not yet approved for younger age-groups.
  • The J&J/Janssen COVID-19 vaccine remains an option for those who cannot receive an mRNA vaccine, but it should be administered only after full informed consent.

The J&J/Janssen COVID-19 vaccine initially looked promising a year ago because of its single-dose primary series and its much less stringent storage requirements. However, things have not quite panned out for the vaccine. Its effectiveness after a single dose has proven to be significantly inferior to the 2-dose mRNA vaccines, and it has now been associated with a very serious, albeit rare, adverse reaction.

The major take-home point for physicians to pass on to their patients is that the nation’s system for monitoring vaccine safety works. It can pick up serious adverse reactions that occur at a rate as low as 1/300,000. This should be reassuring.

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