The COVID-19 vaccines were globally introduced in 2020, accompanied by the most intensive vaccine safety monitoring program the world has ever seen. After three years of administration, data shows that the vast majority of the people who received 13.6 billion doses experience either no vaccine reactions, or reactions that are mild and self-limiting (fever, chills, tiredness, and headache, etc).
How are vaccines monitored for safety?
Anytime a medicine or a vaccine is approved and introduced into wider use, rare adverse events are found in the population that would be impossible to find in clinical trials. For example, if a rare event occurs in 1 in a million people, you would need to enroll a million people into a clinical trial to find just one case of this event. So, monitoring for rare events is a very normal and transparent part of post-approval safety surveillance for any drug, not only for vaccines.
The US government constantly monitors different databases like VAERS for rare adverse events and potential safety signals that may arise, and then analyze whether the number of events among vaccinated individuals is greater than what would be seen normally in the population. It is important to note that while some rare side effects of the COVID-19 vaccines have been shown to occur more frequently than in the background rate of the population, the number of these events represents a tiny percentage of the billions of people overall who received the vaccines.
Vaccine safety monitoring in the US
The public was already aware of safety signals of a few very rare adverse events detected in databases (i.e. rare allergic reactions, myocarditis and pericarditis detected in young men after mRNA and Novavax vaccines, or blood clots seen after the J&J/Janssen vaccine). Even though the blood clots linked to the J&J/Janssen vaccine were extremely rare, this vaccine product was subsequently de-prioritized for use as a result, proof that the safety surveillance system is very sensitive to detect the rarest events, and that the system does work to make vaccines safer.
99 Million-person safety study that came out this week
The largest COVID-19 safety study to date was published online this week in the Journal Vaccine. The study, which was spearheaded by the Global Vaccine Data Network (GVDN) and funded by the US centers for Disease Control and Prevention (CDC), included data on over 99 million vaccinated individuals across 10 sites in 8 countries.
Three vaccines were examined in the study: two were mRNA vaccines (Pfizer/Biotech and Moderna) and one viral vector vaccine (AstraZeneca, which is not used in the US but is the same vaccine type as the J&J/Janssen vaccine).
The study methodology examined the “observed” post-vaccination rates from a pre-specified list of adverse events to the “expected” rates, or the historical, background rate of the same events calculated from the same populations during 2015-2019, before the vaccines were available. Any event where the lowest end of the observed rate range was estimated to be at least >1.5x the expected/background rate (using the lower bound of the confidence interval) was flagged as a potential safety signal.
What did the study find?
Despite the headlines that there was some kind of bombshell this this study, the main, statistically significant findings from this study really only served to confirm safety signals that had already been already found in other databases around the world. The following are the events that were identified as safety signals:
Guillain-Barré syndrome (2.49, 95 % CI: 2.15, 2.87) following the first dose of Astra-Zeneca (viral vector vaccine)
Cerebral venous sinus thrombosis (3.23, 95 % CI: 2.51, 4.09) following the first dose of Astra-Zeneca (these are the blood clots that are also linked to J&J, another viral vector vaccine)
Acute disseminated encephalomyelitis (3.78, 95 % CI: 1.52, 7.78) following the first dose of Moderna (mRNA vaccine) – [NOTE: THIS WAS NOT FOUND IN A MORE RIGOROUS FOLLOW-UP STUDY DONE BY THIS SAME GROUP]
Myocarditis and pericarditis following Pfizer and Moderna (mRNA vaccines), and Astra-Zeneca (viral vector vaccine) were >1.5
All of the other findings of lower magnitude from the study were “hypothesis generating” and should be followed-up on with more rigorous designs.
This was the exact conclusion from the authors -
“This multi-country analysis confirmed pre-established safety signals for myocarditis, pericarditis, Guillain-Barré syndrome, and cerebral venous sinus thrombosis. Other potential safety signals that require further investigation were identified.”
Should I get the vaccine next fall?
The bottom line is that no medication or vaccine is 100% effective or 100% safe, and everything has risks and benefits. But any risks of medications and vaccines need to be balanced against the risks of the disease you are trying to prevent. COVID-19 has killed over a million people in the US and has led to permanent disability in many others. The myocarditis/pericarditis from COVID-19 is 7-10x more common and has a more severe clinical presentation. The vaccines are very safe overall, and the benefit of the vaccine still definitely outweighs the risks.