Monkeypox Vaccine: Expanded Access and Key Facts

Yesterday, the FDA expanded access to the monkeypox vaccine JYNNEOS with a lower-dose option administered just under the skin (intradermal). In November of last year, JYNNEOS was unanimously approved as an alternative to its predecessor smallpox vaccine due to fewer side effects and easier administration. Production of this newer vaccine is being ramped up quickly to combat the monkeypox outbreak.

The monkeypox vaccine has an interesting story rooted in the development of the world’s first vaccine against smallpox. A medical student in the English countryside, Edward Jenner, noticed that milkmaids who had recovered from cowpox did not catch smallpox. He took a bit of virus from a cowpox blister and rubbed it into 8-year-old James Phipps’ skin on May 14, 1796. Phipps got a blister but recovered. Six weeks later, Jenner inoculated Phipps with smallpox and the boy did not get sick. Other vaccines were developed using this model and in 1980 the World Health Organization declared smallpox eradicated.

The older smallpox vaccine used in the US to vaccinate children was highly effective but was phased out in 1971 due to serious side effects that included myopericarditis and transmission to household contacts. Although JYNNEOS is also a live viral vaccine, it is “replication-deficient” unlike its “replication-competent” predecessor. This means the newer vaccine does not reproduce inside the body as the older model did. Until the outbreak of monkeypox in May 2022, the vaccine has been given to people in laboratory research and healthcare who have a higher risk for occupational exposure to orthopoxviruses.

How do we know the new intradermal option works?

The lower dose intradermal option is available for those 18 years of age and older who are at high risk of exposure. The emergency decision to expand access is based on a study of 524 people showing the antibody response was comparable.

How effective are monkeypox vaccines?

The FDA licensed JYNNEOS in 2019 for adults over 18 years to prevent smallpox or monkeypox. The vaccine is administered as a 2-dose series, 28 days apart. The effectiveness of JYNNEOS is based on data from clinical trials which measured immunogenicity—the ability of the vaccine to produce an antibody response. There is no real-world data yet on the effectiveness of vaccination during the current outbreak of monkeypox.

Do they protect from infection or just from complications?

The vaccine is licensed to prevent infection and may lessen the severity of disease if administered within 4 days of exposure. Those with a known recent exposure 4-14 days prior are still encouraged to seek testing and vaccination. A delayed vaccination may not prevent disease but may lessen symptom severity.

What are common side effects of the vaccine?

Side effects of the JYNNEOS vaccine include injection site pain, redness, swelling, itching, fatigue, nausea, chills and muscle aches. The intradermal option produces more redness, firmness, swelling and itchiness at the injection site but less pain than the subcutaneous route. Due to the unknown risk of myocarditis following JYNNEOS vaccination, young men in the highest risk group for this adverse reaction should consider waiting to get an mRNA vaccination if they are receiving JYNNEOS.

Does a smallpox vaccine protect against monkeypox?

Yes, the two viruses are related orthopoxviruses.

Who should get vaccinated and who is eligible for the monkeypox vaccine?

People with a known exposure to monkeypox or those with multiple sex partners in the last two weeks in an area with high monkeypox incidence should seek vaccination. Although JYNNEOS has not been studied in children or adolescents, several children have been vaccinated in 2018-19 in the United Kingdom and during the current outbreak in the US with no serious safety concerns reported. Children who are considered high risk for serious disease can be vaccinated by their clinician through a single use expanded access authorization.

Can people with HIV, a weakened immune system, or skin conditions get vaccinated?

Yes, people with immunocompromised status or skin conditions are eligible to be vaccinated. These individuals may be more likely to develop serious disease if infected, even after vaccination, and they may also be less likely to develop a robust immune response to the vaccine. Those with immunocompromising conditions should also take precautions to avoid infection.

Can people with a heart condition get vaccinated?

Given the theoretical risk of myopericarditis following vaccination with JYNNEOS, people with three or more major cardiac risk factors should discuss risks and benefits of vaccination with their clinician. According to the CDC, major cardiac risk factors include smoking, high blood pressure, diabetes, high cholesterol, and heart disease at age 50 years in a first-degree relative.

Can people who are pregnant or breastfeeding get vaccinated?

The JYNNEOS vaccine has not been specifically evaluated in people who are pregnant or breastfeeding, thus it is unknown whether the vaccine is excreted in breastmilk or if vaccination affects the milk supply. Because the vaccine is replication-deficient, it is not thought to present a risk for pregnant or breastfeeding women and risks versus benefits of vaccination should be discussed with the clinician.

Can I get the vaccine at the same time as other vaccines?

There is no data yet regarding administration with other routine vaccines, but the CDC advises that JYNNEOS may “typically be administered without regard to the timing of other vaccines,” preferably in a different site on the body. Given the theoretical risk of myopericarditis following JYNNEOS, adolescent males should consider separating monkeypox and mRNA vaccination by 28 days.

Where can you get a monkeypox vaccine in the US?

The local health departments are being allocated doses from the federal government. Contact your local health department or search to find a local clinic.

How many doses do people need?

Two doses are administered four weeks apart. The absolute minimum interval between doses is 24 days, allowing a 4-day grace period. If the second dose is delayed it should be given as soon as possible.

Is the vaccine free?

The vaccine is free of charge through the local health department for those meeting the eligibility criteria.


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