(CNN) Vaccine advisers to the US Centers for Disease Control and Prevention say there has been a higher-than-expected number of cases of a heart ailment among young people, most often males, who've recently received their second doses of the Pfizer and Moderna Covid-19 vaccines. The CDC says the reports of the ailment are "rare" and that "most patients who received care responded well to medicine and rest and quickly felt better."
The advisers' statement, posted June 1 on the CDC website, strikes a different note from their statement about two weeks earlier, which said that the rates of myocarditis -- inflammation of the heart muscle -- were not higher among vaccinated people than among unvaccinated people.
The new report comes as the Israeli Ministry of Health finds a "likelihood of a link" between the second dose of the Covid-19 vaccine and myocarditis, most commonly among males ages 16 to 30.
The June 1 report by a work group of the CDC's Advisory Committee on Immunization Practices states that within 30 days of receiving the second dose of either Pfizer or Moderna vaccines, "there was a higher number of observed than expected myocarditis/pericarditis cases in 16-24-year-olds."
This outside group of experts, many of them physicians at academic medical centers, advises the CDC, but doesn't represent the agency itself. The CDC has not said if the number of cases of the heart ailments is higher than expected.
The CDC says on its website that benefits of Covid-19 vaccination outweigh the known and potential risks "including the risk of myocarditis or pericarditis," which is swelling of the tissue around the heart. The agency says it is "actively monitoring these reports, by reviewing data and medical records, to learn more about what happened and to see if there is any relationship to COVID-19 vaccination."
The cases occurred mostly among male adolescents and young adults age 16 years or older, typically within several days after vaccination and more often after getting the second shot than after the first, according to the CDC.
The agency advises people to be on the lookout for certain symptoms following Covid-19 vaccination, such as chest pain, shortness of breath and heart palpitations.
The myocarditis assessments come at a time when the Biden administration has been encouraging young people to get vaccinated to protect themselves and others.
"For young people who may think this doesn't affect you, listen up, please: This virus, even a mild case, can be with you for months. It will impact on your social life. It could have long-term implications for your health that we don't even know about yet or fully understand yet," President Joe Biden said at a White House briefing June 2, urging young people to get vaccinated for themselves and "to protect those more vulnerable than you: your friends, your family, your community."
There's concern the President's effort could be hindered by parental worries over the risk of myocarditis following vaccination.
Medical groups, such as the American Academy of Pediatrics and the American Heart Association say even if there is a very small risk of getting myocarditis after vaccination, it is heavily outweighed by the risk of complications from Covid-19.
"Young people need to be protected, and they also need to not be a reservoir for the virus," said Dr. Nelson Michael, director of the Center for Infectious Diseases Research at the Walter Reed Army Institute of Research, noting that his son and daughter, who are in their 20s, were vaccinated against Covid-19.
The CDC has reached out directly to state health departments and medical societies, such as the American Academy of Pediatrics and other groups about the myocarditis reports. The agency has also issued several statements on its website in the past few weeks about myocarditis following the two mRNA vaccines, Pfizer and Moderna, including one for physicians and one for the public.
Aside from these pages, CDC officials have not directly communicated to the public about any possible risk of myocarditis with the vaccines. CDC declined CNN's request to speak with an expert, instead issuing a statement by a spokesman.
"In early May, CDC first received reports from these systems about cases of myocarditis and pericarditis being reported in the United States after mRNA COVID-19 vaccination," according to the statement by agency spokesman Jason McDonald. "Given the risk of COVID-19 infection in adolescents, CDC continues to strongly recommend that adolescents age 12 to 17 get vaccinated."
Jerica Pitts, a spokeswoman for Pfizer, said the company is aware of the myocarditis reports, and that "a causal link to the vaccine has not been established" and that "with a vast number of people vaccinated to date, the benefit risk profile of our vaccine remains positive."
A Moderna spokesman did not respond to a request for comment to this story.
The increase in myocarditis cases could be, at least in part, because the CDC and the American Academy of Pediatricians prompted doctors to be watchful for such cases.
But a source familiar with the situation told CNN it's appearing more and more likely that there is a real link between the vaccine and the cases.
"It looks like this is a biological phenomenon rather than a chance event," the source said. "It looks to be a pattern and not just random."
"They're not sure yet whether there is a causal association, but they're keeping a close eye on this," the source said. "They're open to the notion that this may be a causal situation, but the case is not conclusive yet, and it's certainly not enough to change their recommendations -- they will continue to recommend very clearly that everyone over age 12 should get the vaccine."
In February, Israeli physicians reported the case of a 19-year-old man hospitalized with myocarditis five days after receiving his second dose of the coronavirus vaccine. The Jerusalem Post first reported the case, and the details of the article were confirmed to CNN by Natan Applebaum, chief executive officer of Terem, a chain of Israeli emergency clinics, where the man received care.
In March, the US Department of Defense started to receive reports of myocarditis among vaccinated military health patients, according to Military.com. As of June 9, the department has identified approximately 30 cases of myocarditis among the more than 3.6 million doses administered, according to department spokeswoman Lisa Lawrence.
In April, Spanish doctors published a report in a medical journal of a case of myocarditis in a 39-year-old physician with underlying health problems following his second dose of a Covid-19 vaccine.
In the United States, if anything goes wrong after vaccination -- any vaccination, not just against Covid-19 -- doctors and patients are encouraged to report it to the Vaccine Adverse Event Reporting System, a database managed by the CDC and the US Food and Drug Administration.
Once problems are reported, the next step is to assess whether they occurred by chance or if the vaccine is a possible cause.
First, experts have to look at the reports one by one to see what happened to the patient; reports of a particular illness might turn out to be something else or perhaps nothing at all.
Then biostatisticians and epidemiologists have to determine whether the illnesses are linked to the vaccine or just a coincidence. To do this, they compare how often the adverse event happened to people following Covid-19 vaccination versus unvaccinated people around the same age.
On May 20 a group of CDC vaccine advisers posted a report on the CDC website that there had been "relatively few reports" of myocarditis after vaccination.
In that May report, the CDC advisers wrote that the "rates of myocarditis reports in the window following COVID-19 [with Moderna and Pfizer] have not differed from expected baseline rates," indicating that the number of reports was not higher than what would be expected among unvaccinated people. The advisers continued to direct doctors to report cases of myocarditis following coronavirus vaccination.
It's not clear what changed by June 1, when the CDC advisers reported that myocarditis cases following vaccination in the 16-to-24 age group were higher than expected.
Another CDC system, the Vaccine Safety Datalink, uses health information from nine US medical centers' reports to monitor vaccine safety and conduct studies about rare and serious adverse event after immunization.
This system did not find that recently vaccinated people were more likely to get myocarditis, but "analyses suggest that these data need to be carefully followed as more persons in younger age groups are vaccinated," according to the vaccine advisers' report.
A commentary published last week in the medical journal Pediatrics reviewed a published account of seven cases of myocarditis in teen males following Covid-19 vaccination.
The authors concluded that "there are some concerns regarding this case series that might suggest a causal relationship" between the vaccine and myocarditis.
"There are some suggestions [the link] may be real, but it's not definitive yet," Dr. Sean O'Leary, a co-author of the commentary, told CNN.
O'Leary, a pediatric infectious disease specialist at University of Colorado Medicine and Dr. Yvonne Maldonado, a pediatric infectious disease specialist at Stanford Medicine, wrote that there were several reasons there could be a link.
The seven cases, which occurred at five medical centers among otherwise healthy males ages 14 to 19, had certain features in common.
All of the patients developed symptoms within four days after receiving the second dose of Pfizer's Covid-19 vaccine.
"The consistent timing of symptoms in these seven cases after the second vaccination suggests a uniform biological process," O'Leary and Maldonado wrote.
The patients had chest pain and five of them had fevers. They were hospitalized for between two and six days and their illnesses were "mild," responding "rapidly" to medications.
"While the authors are quick to point out that a causal relationship between vaccination and myocarditis has not been established, the temporal association of these cases with vaccination as well as the striking similarity in the clinical and laboratory presentations raise the possibility for such a relationship," O'Leary and Maldonado wrote.
They added that "a causal association, if it exists, is likely extraordinarily rare."
While the cause of a patient's myocarditis is often never identified, when it is known, an infection is often the culprit. O'Leary and Maldonado wrote that a thorough diagnostic workups on the seven patients failed to find an infectious cause, and they also noted the "dearth" this year of common respiratory viruses known to cause the heart condition.
The Israeli Ministry of Health has released the most detailed analysis yet of cases of myocarditis following Pfizer vaccination. In a June 2 statement to the press, the ministry reported 27 cases of myocarditis following the first dose of the vaccine out of 5.4 million vaccine recipients, and 121 cases after the second dose out of about 5 million vaccine recipients.
"To date, the phenomenon has been reported mainly among young men, especially aged 16-19, usually after the second dose of the vaccine. Most cases are hospitalized for up to 4 days only and 95% are defined as a mild illness," according to the statement. "There is a likelihood of a link between receiving a second dose of vaccine and the onset of myocarditis in young men aged 16-30, and the link is stronger in young people aged 16-19 relative to other ages. The relationship weakens with increasing age. In most cases, it is a mild illness that passes within a few days."
The Israeli health officials did not specify how much more common myocarditis was among the vaccinated people compared with the rest of the population. The authors of the article about the US cases said that the incidence of myocarditis "is unknown and varies by season, geography, and age."
After a "lengthy discussion," Israeli health experts decided on June 1 to allow vaccination of adolescents aged 12 to 15, noting that the risks of complications from Covid-19 are higher than the risk of receiving the vaccine.
Less than a month after becoming eligible, nearly a quarter of adolescents in the US ages 12 to 15 have received at least one shot.
But there's concern among public health experts that upon hearing of this possible myocarditis link, parents will be hesitant to take their children to be vaccinated.
"This could put the kibosh on adolescent vaccination -- everyone is concerned about it," said the source who is familiar with the situation.
There's an indication that parents have already become less enthusiastic about the vaccine.
News that CDC was investigating the myocarditis reports first appeared in the general media on May 22, with publication of a story by the New York Times, followed by stories in other media outlets.
According to a CNN analysis of CDC data, Covid-19 vaccinations for adolescents 12 to 15 years old had been rising steadily since May 12, when the CDC gave the green light for shots for that age group, but then started to decline the day after the New York Times article appeared.
The seven-day averages of first doses administered to this age group peaked at 220,401 on the day the Times article published. Then, for the first time since the CDC authorized the shot for 12 to 15 year olds, the numbers started to drop steadily, going down to 75,168 less than two weeks later on June 3, the most recent day for which data is available.
The media articles are not the only possible reason for the drop in enthusiasm for vaccinating young people, but the timing of the marked decline so early on in the rollout for this age group suggests the stories played a role, said David Holtgrave, who worked on infectious diseases at CDC for seven years.
Holtgrave told CNN in an email that vaccinations for this age group "seem to slow substantially when it was reported in the media that [the CDC advisers] and CDC expressed a need to closely monitor myocarditis in young persons after COVID vaccination."
He added that CDC could do more to increase parents' confidence in the vaccine.
While the agency has communicated directly to public health departments and professional organizations, such as the American Academy of Pediatrics, the only statements to the public have been on the agency's website.
Holtgrave said regular statements from CDC officials directly to parents on the possible myocarditis risk could go a long way toward increasing public confidence.
"It is useful for CDC to frequently, proactively describe in detail how it is being aggressively vigilant to monitor for vaccine-related consequences. The better this aggressive monitoring is articulated to the public, the more we can have a continued, high-level of confidence that the vaccine rollout is indeed going very well and consequences such as side effects or breakthrough infections are minor and occur at reassuringly very low levels," wrote Holtgrave, the dean of the School of Public Health at the University at Albany.
Dr. Paul Offit, a pediatric infectious disease expert at the University of Pennsylvania, agrees.
"How about a little more information about the data? Who's looking at this? What's their process for looking at it? When will we know more?" he said. "This has started to worry people, and the lack of details doesn't help. It's anxiety producing."
In his statement, CDC spokesperson Jason McDonald said "CDC is communicating in the most appropriate way in light of the actual risk of an adverse event based on the data received to date."
O'Leary, the co-author of the Pediatrics commentary, said he thought the CDC had to "walk a fine line." On the one hand, the agency has to be transparent about the myocarditis reports, but they also have to do their best to make sure parents aren't scared away from the life-saving vaccine.
"If they communicate this too broadly, it could create a public scare and cost lives," he said. "That's what they have to contend with, and I don't want to second guess how they've communicated this. I'm sure they've been thoughtful about it because there's a big downside if they do it wrong."
Offit said the key to communicating any potential risk of myocarditis is to emphasize that children can and do sometimes die from Covid-19, and that the risk of that happening is larger than any possible risk of getting myocarditis from the vaccine.
"I would vaccinate my teenager in a second. This is an extremely rare risk," Offit said.
Offit praised CDC Director Dr. Rochelle Walensky for her description last week at a White House briefing of "troubling data" on hospitalizations of adolescents with Covid-19, especially those who needed to be admitted to the intensive care unit and put on ventilators.
"She's very emotional -- you can see it in her face, she's pleading, saying this can be a serious disease in children and here's how to prevent it. She comes off as a parent, which she is. She looks like a real person," Offit said.