Why Young Males Experienced Higher Rates of Vaccine-Associated Myocarditis

Understanding the biological differences behind the signal

During the COVID-19 vaccination campaigns, safety monitoring systems in several countries identified a rare but consistent pattern: myocarditis (inflammation of the heart muscle) occurred more frequently in younger males after mRNA vaccination, particularly after the second dose.

The signal was detected independently across multiple countries and surveillance systems, including those run by the Centers for Disease Control and Prevention and European and Canadian public health authorities.

Our own CVTAG knew about the safety signals coming out of Israel as early as 22 June 2021. (Meeting Minutes via Wayback).

By July 2021, they had more data, on cases of Myocarditis reported after vaccination in NZ.

In Aotearoa New Zealand, as of 01 July 2021, 10 reports of myocarditis and/or pericarditis
occurring in individuals following vaccination with the Pfizer COVID-19 vaccine have been
received by Centre for Adverse Reactions Monitoring (CARM). Of the 10 cases, 4 were male, 2
were in individuals less than 30 years old (1 male, 1 female), and the ages of the individuals
ranged from 24-63 years. Reported time from vaccination to onset was up to 18 days, with 2
cases occurring after the first dose and 8 after the second dose. All cases have been medically
assessed by CARM and follow-up information sought.

Whilst the New Zealand data do not currently indicate an association between the Pfizer COVID-19 vaccine (Comirnaty) and
myocarditis, the international data does. Therefore, Medsafe has confirmed that Pfizer will
update the data sheet for the Pfizer COVID-19 vaccine.

They made some recommendations:

CV TAG recommends that:
a) People aged 16-29 years receive their second dose of the Pfizer COVID-19 vaccine at
least 8 weeks after the first dose. A longer interval between doses may reduce the
frequency of some side effects while conferring robust protection from COVID-19.

b) People aged 12-29 years who require regular clinical review by a cardiologist are
advised to discuss the risks and benefits of the COVID-19 vaccine with their healthcare
team in order to plan a vaccination schedule that best supports their needs.

c) People aged 30 years and over should still receive two doses of the vaccine, at least
21 days apart. Myocarditis and/or pericarditis after vaccination in this group is rare,
and the risks of severe disease and sequelae due to COVID-19 are substantially higher
in older compared to younger age groups.

d) Anyone who develops confirmed myocarditis and/or pericarditis after the first dose
should not receive a second dose of the Pfizer COVID-19 vaccine. CV TAG will consider
alternative options for a second dose of COVID-19 vaccination in this group at a
future date as evidence emerges from overseas safety monitoring sources.

e) If, after discussion with their health care provider, the individual and/or their whānau
decides that the benefits of receiving two doses and gaining robust protection against
COVID-19 sooner, outweigh the potential risks, then the individual may receive the
second dose as per the current indication.

One of the most striking features of the data was not just that myocarditis occurred — but that the increase was strongly concentrated in males under about 30 years old, while females of the same age showed little or no corresponding increase.

Understanding why this occurred requires looking at biological differences between males and females in immune response, hormones, and baseline myocarditis risk.

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Myocarditis already occurs more often in males

Long before COVID-19 vaccines existed, myocarditis was known to occur much more frequently in males than females.

Medical literature consistently shows that:

• Myocarditis is 3-6 times more common in males

• The highest incidence occurs in teenagers and young adults

Common causes include viral infections such as enteroviruses, adenoviruses, and influenza.

This means the vaccine-associated cases were not appearing in a completely new population. Rather, the cases appeared within a group already known to be biologically more susceptible to myocarditis.

Hormones play a major role

One of the key biological differences involves sex hormones, particularly testosterone and estrogen.

Testosterone (higher in males)

Testosterone tends to promote pro-inflammatory immune responses.
In myocarditis research, testosterone has been shown to:

• increase inflammatory T-cell activity

• amplify immune responses targeting heart tissue

This makes inflammatory injury to heart muscle more likely in males during strong immune activation.

Estrogen (higher in females)

Estrogen appears to have protective effects on cardiac tissue.

Research suggests estrogen:

• reduces inflammatory signaling in heart muscle

• enhances tissue repair

• promotes regulatory immune responses

This protective effect likely explains why females experience much lower myocarditis rates overall.

Female immune systems regulate inflammation differently

Another important difference is how the immune system behaves.

In general, females tend to have:

• stronger antibody responses to vaccines

• stronger immune regulation mechanisms

This combination means that while females often mount stronger immune responses, those responses are better controlled and less likely to cause inflammatory tissue damage.

Males, by contrast, may experience more aggressive inflammatory responses with weaker regulatory control, increasing the likelihood of myocarditis during intense immune activation.

Genetics also influence immune regulation

Many immune-related genes are located on the X chromosome.

Because females have two X chromosomes, they have:

• greater diversity in immune gene expression

• stronger immune regulatory pathways

Even though one X chromosome is largely inactivated, parts remain active, providing additional immune system redundancy.

This genetic factor may further reduce the risk of uncontrolled inflammation in females.

Why the second dose showed the strongest signal

Safety monitoring systems consistently found that myocarditis cases were most common after the second dose of mRNA vaccines.

This is because the second dose acts as an immune system re-challenge.

After the first dose, the immune system becomes primed.
The second dose triggers:

• rapid antibody production

• strong T-cell activation

• increased cytokine signaling

In most people this produces a normal immune memory response.
In a small number of individuals — particularly younger males — the amplified immune reaction can briefly affect heart tissue, resulting in myocarditis.

Dose spacing turned out to matter

Another important discovery was that shorter intervals between doses increased the myocarditis risk.

Early vaccination schedules typically used a 3-4 week gap between doses because that was the interval used in the clinical trials.

However, later safety analyses showed that longer spacing reduced myocarditis rates, particularly in younger males.

For example:

This pattern was observed in multiple countries, including Canada and several Nordic nations such as Sweden and Finland.

In response, some public health authorities recommended longer dose intervals for younger populations to reduce the risk.

Vaccine formulation also played a role

Another finding from safety surveillance was that myocarditis rates differed slightly between the two major mRNA vaccines.

Studies found that myocarditis occurred more frequently with the vaccine produced by Moderna compared with the vaccine produced by Pfizer.

One possible explanation is that the Moderna formulation originally used a larger amount of mRNA per dose, which may produce a stronger immune response.

What the data ultimately showed

Across millions of vaccine doses, the overall pattern was consistent:

• When it occurred, it was concentrated in young males

• Cases were most often seen after the second dose

• Longer spacing between doses reduced the risk

These findings highlight how biological differences between males and females can influence how the immune system responds to vaccination.

They also demonstrate how large-scale safety monitoring systems were able to detect and respond to rare side effects during the global vaccination campaign.

The broader lesson

The myocarditis signal illustrates an important reality of medicine:

Biology is not identical across populations.

Age, sex, genetics, and immune system differences can all influence how individuals respond to vaccines, medications, and infections.

Understanding those differences allows health authorities to refine recommendations and improve safety over time.

…assuming their recommendations are received by, and implemented by Ministers.  Not doing that is what has landed NZ in the position we are in.