The rare cases of myocarditis among recipients of mRNA COVID-19 vaccine—mostly in young men—are caused by a generalized immune-cell and inflammatory response rather than vaccine-triggered antibodies, suggests a small study published late last week in Science Immunology.

Yale University researchers analyzed immune responses among 23 participants who developed post-vaccination myocarditis (generally mild, transient inflammation of the heart muscle) and healthy vaccinated control patients. Most myocarditis patients (87%) were boys or young men (average age, 16.9 years) and were generally healthy before vaccination.

Blunted SARS-CoV-2 antibody response

No participants had evidence of previous SARS-CoV-2 infection. Most myocarditis patients began experiencing symptoms 1 to 4 days after the second dose of the Pfizer/BioNTech mRNA COVID-19 vaccine. Symptoms included chest pain, heart palpitations, fever, shortness of breath, sweating, headache, muscle pain, fatigue, and nausea.

Lab tests among the myocarditis patients showed elevated levels of biomarkers of inflammation of the heart, heart damage, and systemic inflammation. "The immune systems of these individuals get a little too revved up and over-produce cytokine and cellular responses," senior author Carrie Lucas, PhD, said in Yale press release.

The immune systems of these individuals get a little too revved up.

Electrocardiographic findings included pericarditis (inflammation of the lining of the heart), and echocardiographic results showed borderline-low or reduced left-ventricle ejection fraction, a measure of the heart's ability to pump blood. These patients didn't have higher levels of SARS-CoV-2–specific or neutralizing antibodies but rather had similar or lower levels than controls.

Cardiac magnetic resonance imaging (CMR) revealed abnormalities consistent with myocarditis and/or pericarditis. These patients received nonsteroidal anti-inflammatory drugs, and some also received steroids and intravenous immunoglobulin (antibodies). Patients were released after 1 to 6 days, but most still had imaging abnormalities on CMR at 2 or more months.

The risk of COVID-19 vaccine–associated myocarditis is very rare, affecting about 22 to 36 in 100,000 12- to 17-year-old boys who received their second dose no more than 21 days earlier, according to the Centers for Disease Control and Prevention. In fact, myocarditis is more common after COVID-19 infection among unvaccinated boys, at 50 to 65 cases per 100,000.

The Chicago area is experiencing a resurgence in mpox infections, which are at the highest level since November and mark the highest weekly case rate in the nation, Howard Brown Health said in a May 5 press release.

The agency, which serves the LGBTQ+ community, said it has confirmed seven cases since Apr 17, with results of other tests still pending. It added that, in the 3 previous months, the Chicago Department of Public Health had reported only one mpox case.

Patrick Gibbons, DO, Howard Brown’s chief medical director, said, "We urge sexually active members of our community to receive the mpox vaccine. For example, unvaccinated people planning to attend International Mr. Leather at the end of May should receive their first dose of the mpox vaccination as soon as possible." He added that increased vaccination rates will better protect the community from another outbreak.

Concerns over global resurgence

Following last year's international outbreak, cases dropped sharply over the winter months. In February, the World Health Organization kept the public health emergency of international concern (PHEIC) in place for mpox, citing sustained transmission in some countries, undetected transmission and under-reporting in others, and concerns about a possible resurgence when social events and other mass gatherings resume.

Other countries have reported upticks in mpox activity, such as a cluster in France and South Korea, where officials last month raised the alert level from 1 to 2.

An Australian postmarketing study of adverse events following Jynneos mpox vaccination finds that local adverse event rates were highest following intradermal administration, but absolute event rates were lower than in previous studies, and the vaccine was well tolerated overall. The study was published late last week in JAMA.

In late summer of 2022, Australia began to use the Jynneos vaccine with a dose-sparing schedule of 0.1-milliliter (mL) intradermal vaccine recommended for preexposure to mpox, and 0.5-mL subcutaneous vaccine for postexposure prophylaxis (prevention)—two doses given 4 weeks apart for both approaches.

The study was based on the results of a survey given to 13,306 participants. The median age was 41, and 97% of participants were men. The participants received the vaccine from August 8, 2022, to March 20, 2023, and were asked about adverse events 0 to 7 days after the second dose.

This study also found a low percentage of people reporting medical review [0.7% to 1.2% of all recipients] or missing daily activities [2.5% to 3.7%].

Vaccinations were classified as intradermal if administered in the forearm and subcutaneous if administered in the deltoid. According to the authors, the adverse event rate was highest following dose one of intradermal vaccination (53%) and lowest following dose two of subcutaneous vaccination (31%). For both routes, dose one produced more non-severe localized reactions, including redness, itching, and swelling.

"Absolute event rates were lower than in previous studies, which reported more than 30% systemic adverse events following both routes and local adverse event rates of more than 50% following subcutaneous administration and nearly 100% following intradermal vaccination," the authors said. "This study also found a low percentage of people reporting medical review [0.7% to 1.2% of all recipients] or missing daily activities [2.5% to 3.7%], suggesting that the vaccine is generally well-tolerated."

In an update last week on the intensifying chikungunya outbreak in the Americas, officials from the Pan American Health Organization (PAHO) said they are seeing changes in the timing and magnitude of cases that are likely related to the effects of climate change and unplanned urbanization.

PAHO first warned about a new surge of cases in January, and since the first of the year, more than 214,000 cases have been reported in five countries, with Paraguay hit the worst. Argentina and Uruguay have reported their first local transmission of the mosquito-borne virus, and Bolivia is reporting high numbers of chikungunya and dengue infections.

In a May 4 statement, PAHO said unusually high temperatures, anticipated or increased rains, and humid conditions have allowed Aedes mosquitos to survive in areas where their populations weren’t supported before. Also, increased or unplanned urbanization have increased watering spots at households, a factor that creates a breeding environment for the mosquitoes.

Seven vaccines are in development, with three in clinical trials. PAHO said the best protection for now is to use insecticide and mosquito netting and curb breeding sites. Though few deaths are reported, chikungunya virus can cause debilitating joint pain that can last as long as 6 months.