A standard way to determine whether there is a genuine concern that a new vaccine may be causing a particular type of serious adverse event (SAE), such as 'myocarditis', is to compare the proportion of adverse event reports associated with the new vaccine that include myocarditis with the proportion that include myocarditis in other vaccines.
For example, suppose that in Vaccine Adverse Event Reporting System (VAERS): For the new vaccine there are 1000 reports of SAEs of which 300 include myocarditis.
For all other vaccines there are 2000 reports of SAEs of which 100 include myocarditis. Then 30% of the reports for the new vaccine include myocarditis compared to just 5% for all other vaccines. This is used to calculate the proportional reporting ratio (PRR) which is simply 30% divided by 5% in this case, and here the PRR is equal to 6 for a myocarditis SAE. The fact that there is a much greater proportion of myocarditis events in the new vaccine compared to previous vaccines should be concerning.
To be considered a potential 'safety signal' by the Centers for Disease Control and Prevention (CDC), the PRR must be at least 2 and must also have a chi-square test statistic value of at least 4. The chi-square test statistic is a classical statistical test of significance that takes account of the total number of events and reports.
In this example the chi-square is 361 which is highly significant; but if, say we divide the values in the above example by 100 (so 3 out of 10 compared for the new vaccine and 1 out of 20 for all other vaccines), then although the PRR is still 6, the chi-square value is just 1.9 which would not be significant under the CDC's rule.
The CDC conducted a PRR analysis of adverse events reported in the VAERS system for the Pfizer and Moderna covid vaccines for the period Dec. 14, 2020, to July 29, 2022. For each different type of adverse event reported, the CDC calculated the PRR of every SAE for each of the two covid vaccines against all other non-covid vaccines. They also computed the PRR of every SAE for Pfizer against Moderna....<<<Read More>>>...
