Hi Michelle,

The vaccine virus in Jynneos is incapable of replicating in mammalian cells so there is theoretically zero risk of infant exposure to the vaccine here, but breastmilk studies have not been done in animals or humans to confirm this.

The myocarditis risk that was seen after ACAM2000 smallpox vaccine was about 5.7 cases/1000, it was largely seen in younger males because the largest application of smallpox vaccination post-eradication of the wild type infection was in military recruits. And this demographic seems to have the greatest propensity for myocarditis after an immune stimulus.

Even though the Jynneos virus can’t replicate, it provokes the same immune response as ACAM2000, so there was concern it might carry the same risk of myocarditis. Therefore in phase 2 and 3 trials the investigators did extensive monitoring for cardiac side effects. There was a very low signal for asymptomatic elevations of troponin (1.3% for Jynneos, 0.2% for placebo). None of these patients had clinically significant events or needed care, but it suggests that there is likely some risk of myocarditis from that immune reaction. The risk is low and very likely will end up being greatest in that young adult male population (as we saw with COVID vaccine as well.)

So, essentially zero risk of transmission to the breastfeeding infant, for starters. If there were some freak transmission, oral inoculation is not known to provoke an immune response. Even if it did the risk of clinically significant SE in are essentially nil.

The egg allergy precaution is an extremely cautious stance IMO, because the vaccine is never exposed to egg albumin. It is cultured in fibroblast cells. I would bet that the egg allergy precaution will ultimately be removed but there was extreme caution with the expansion of use to this wide population for the MPX outbreak. From the FDA EUA fact sheet here are the ingredients and excipients in Jynneos, and as you’ll see there is no egg albumin:

WHAT ARE THE INGREDIENTS IN JYNNEOS? JYNNEOS is a live virus vaccine. JYNNEOS contains Modified Vaccinia Ankara-Bavarian Nordic (MVA-BN), a weakened, non-replicating orthopoxvirus. It also contains Tris (tromethamine) and sodium chloride, and may contain small amounts of DNA and protein from the Chicken Embryo Fibroblast cells used to grow the vaccine virus, benzonase, gentamicin, and ciprofloxacin

Hope this helps,

Melanie

Melanie Swift, MD, MPH

(she/her)
Vice Chair, Division of Public Health, Infectious Diseases and Occupational Medicine

Medical Director, Mayo Clinic Physician Health Center

Associate Medical Director, Occupational Health Service

_______________________________
Mayo Clinic
200 First Street SW
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From: MCOH-EH <mcoh-eh-bounces@mylist.net> On Behalf Of Gochnour, Michelle K. via MCOH-EH
Sent: Wednesday, September 21, 2022 12:12 PM
To: MCOH/EH <mcoh-eh@mylist.net>
Cc: Gochnour, Michelle K. <michelle.gochnour@seattlechildrens.org>; Ayres, Katlyn <Katie.Ayres@seattlechildrens.org>
Subject: [EXTERNAL] [MCOH-EH] Question re: JYNNEOS vaccine and breastfeeding

Wondering how any of you are approaching questions regarding breastfeeding and the JYNNEOS vaccine.  We had a situation come up where the child who is breastfeeding has eosinophilic esophagitis (EOE) and a severe allergy to eggs;  and should one be concerned about chicken protein exposure impact on EGG allergy through breast milk? 

Also, the JYNNEOS package insert and June 3 MMWR states data are not available to assess the safety of JYNNEOS in breastfed infants, but goes on to state that because the vaccine is replication-deficient, it likely does not present a risk of transmission to breastfed infants and can be administered to women who are breastfeeding if vaccination is critical.   (Note: the person in question is a researcher planning to work with MPXV positive patient samples and our policy is to require vaccination for research work).  Further, myocarditis has mainly been seen in young adult/adolescent men post COVID-19 vaccination and there is an unknown risk of myocarditis post JYNNEOS vaccine.  Given this logic, would you agree that the risk for myocarditis to INFANT males who have received mRNA COVID-19 vaccine and are breastfeeding with a mother who receives JYNNEOS vaccine is low to none?

Thanks for any/all thoughts!

Michelle Kom Gochnour

Senior Occupational Health & Safety Consultant, Research | Environmental Health and Safety

Seattle Children’s

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