Alex Berenson alleged in tweets that, according to newly released CDC report, being post-second dose [of Pfizer's COVID-19 vaccine] carry significantly increased risk of heart problems for under 25 year olds.
Do young Pfizer COVID-19 vaccine recipients have 200x the background rate of myocarditis/pericarditis?
No. Myocarditis/pericarditis cases appear in male Pfizer COVID-19 vaccine recipients between 16 and 24 years old at 5 to 25 times the background rate, not 200 times.
How the claim arose: Among males 18-24 years old, 219 cases of myocarditis/pericarditis were reported to the CDC within 7 days of receiving a second mRNA vaccine dose (page 28 here). The expected number of myocarditis/pericarditis cases in this population of 18-24 year old males is somewhere between 1 and 8 cases, also in the same CDC source. Alex Berenson arrives at his claim by dividing the 219 cases with the lowest value in the range, a single expected case (i.e. dividing 219 by 1 and using that to say cases appear at over 200 times the background rate). A more accurate method may be to divide by the midpoint of the range (e.g. as I have done here).
What do scientific studies suggest? Studies on myocarditis among young men after receiving an mRNA vaccine dose are minimal. However, in addition to the ongoing CDC investigation, an Israeli study has already been conducted on this subject. In this study, researchers reviewed 275 cases of myocarditis out of 5.4 million people who have received a first dose and 5 million people who have received a second dose.
Ninety percent of the cases picked up in Israel appeared in men, and although myocarditis is normally more common among young men, the rate among those vaccinated was somewhere between five and 25 times the background rate, the report says.
tl;dr: The author of those tweets misinterpreted the CDC presentation. Yes, the presentation states that there is a notable increase, but it is only about 1/10th of what the tweet author claims.
The CDC presentation lists numbers for the expected and actual rate of myocarditis and pericarditis for various age-groups, for male and female patients, and for 7-days and 21-days after receiving a dose and for the 1st or 2nd dose. The worst-looking statistic is that for 7-days after the second dose, which state:
So if we cherry-pick the group with the worst increase - males aged 12-17 - and we assume that the middle value of 0-4 expected cases is 2 cases, then the increase is not 200x the background rate but 64x the background rate. If we take the claim in the tweet above and add all the numbers for "people under 25", then we have between 2-20 expected cases vs. 229 cases. Assuming a middle value of 10 expected cases, that would mean the increase for people under 25 is not 200x but only 23x the expected rate.
And this is still a very low risk. 229 cases of 14,8 million doses administered means approximately one case in 65,000.
Further, the condition is not as bad as it sounds. When the above CDC report was made, of 484 preliminary cases were reported, 323 were confirmed and 148 still under review. Of those 323 confirmed cases, 309 were hospitalized. But 295 were already discharged with 218 reporting full recovery. 9 are still hospitalized (2 in intensive care). There is no data known for 5 cases. There are no reported deaths among those cases so far.
By the way: Some people might wonder how the risk of getting a heart inflammation compares to the risks of dying from COVID-19. It's impossible to predict how the risk of catching the virus will develop in the next months, and it will differ a lot by region. But if you do catch it: the mortality rate of COVID-19 for the age group of 12-30 is about 0.2%, or one death in 500.
The ratio of Observed to Expected counts ranges from 219 to 27.3. The claimant uses this data to suggest the risk ratio may be over 200x.
This is a somewhat sensational interpretation of the data, as it cherry-picks the most extreme end of the most extreme ratio among dozens of choices (there are several other similar tables in the slide deck). The claimant has no fewer than 56 choices of ratios to examine in the CDC slide deck, but doesn't do any kind of multiple hypothesis correction. If you examine enough ratios, you'll eventually find one that shows what you want by chance alone, but is actually unrelated to the factor you're studying (obligatory XKCD).
There's also an issue with sample size, since the expected number of events is very small and highly variable. With little data, statistical estimates have little power and wide confidence intervals. If you collect a very small number of samples, for example, you might conclude that your point estimate shows no significant association with risk, but that your confidence interval ranges from 1000x risk to 1/1000x risk. The fact that your factor "may yield up to 1000x risk" just indicates that you don't have enough data to rule out the possibility, but it's not actually evidence that the factor does yield 1000x risk.
The "best estimate" in the middle of the expected range yields roughly a 50x risk, but the confidence interval suggests it could be as high as 200x or as low as 25x. The ratio "could be as high as 200x" simply because it hasn't been studied very well. With even less data, the ratio "could be" even higher - with insufficient data, we can't statistically rule out the possibility. Had the sample size been small enough to yield a lower expected count of 0, even a single observed case would imply that the risk ratio "could be" arbitrarily high.
Using the "best estimate" is a more reasonable approach, since that should not change with more data (assuming a good sampling procedure), while the extreme ends of the confidence interval will be highly dependent on how many people you've sampled. Assuming new data continues to follow the same pattern, it will become increasingly unlikely that the true ratio is higher than 200x as more data is collected.
Overall, the claimant isn't wrong per se (at this time, it is statistically feasible that the risk ratio for that particular group is higher than 200x), but it says more about the availability of data than the risk posed by the vaccine. There is evidence that the vaccine increases risk of myocarditis/pericarditis, but it's quite unlikely to be as high as a 200x risk.