Like long COVID, post-COVID vaccination syndrome is characterized by a wide variety of symptoms and clinical pictures including chronic fatigue syndrome (CFS/ME), migraines, muscle pain or cardiovascular diseases.

As multifaceted as the symptoms may be, they have one thing in common: They can occur in those affected shortly after COVID vaccination.

Many people think they have post-COVID vaccination syndrome because they developed such symptoms after their shot.

From the point of view of those affected, this is completely understandable, says Harald Prüss from Berlin's Charité hospital and the German Center for Neurodegenerative Diseases (DZNE). But, says Prüss, just because the symptoms occurred after the shot doesn't mean it caused them. "Post-COVID vaccination syndrome is totally overestimated."

By October 31, 2022, almost 51,000 suspected cases of serious side effects after the COVID vaccination had been reported to the Paul Ehrlich Institute, the federal authority responsible for vaccines and medicines in Germany.

The institute has published regular safety reports on COVID vaccines but experts like Prüss believe the vast majority of these cases are due to something other than vaccine side effects.

International body says "no evidence" of COVID vaccines causing severe illness

In early July 2023, the International Coalition of Medicines Regulatory Authorities (ICMRA) said "there is no evidence that COVID-19 vaccines are causing excess mortality. In fact, COVID-19 vaccines have saved millions of lives."

The ICMRA, a grouping of 38 medicines regulatory authorities, had issued a public statement, citing "evidence from the more than 13 billions of vaccine doses given worldwide."

Its members include the European Medicines Agency, the US Food and Drug Administration and national bodies in Africa, Latin America and Asia. The World Health Organization has observer status.

"There is no evidence to indicate that COVID-19 vaccination causes immune impairment," said the ICMRA statement. "Some information circulating on social media has claimed that COVID-19 vaccines could be associated with long COVID. There has been no safety signal from the very large body of data held by international regulators suggesting that long COVID is a possible side effect of COVID-19 vaccination."  

Vaccine side effects not unusual

It is not uncommon for vaccines to cause negative side effects for a small number of people. For example, studies show the H1N1 (swine flu) vaccine as well as the illness itself caused narcolepsy — a chronic inability to regulate sleep-wake cycles — in a very small number of patients with similar, predictable genetic factors.

This response to the vaccine was easier to prove, said Prüss, because fewer people were actually contracting H1N1 at the time. This wasn't the case with COVID, when millions of people were becoming infected — and getting vaccinated — more or less at once.

How can post-COVID vaccine syndrome be diagnosed?

Prüss sees many people who believe they are suffering from post-COVID vaccine syndrome during the neurological post-COVID-19 consultations he conducts at the Charité's neurology clinic.

The first indication that a person's headache, fatigue or muscle weakness could actually have something to do with the vaccination is the time frame: If the symptoms appear a few days to weeks after the injection, there could be a connection he says.

But even then, post-COVID vaccine syndrome is nearly impossible to prove, says the neurologist. "There is not a single biomarker that is widely accepted by science."

A biomarker might be a specific antibody that the body produces in response to vaccination. , For example, researchers discovered a special antibody in the blood of people who had developed inflammation of the heart muscle (myocarditis) after getting vaccinated. Myocarditis is considered a possible, rare side effect after vaccination with BioNTech/Pfizer and Moderna mRNA vaccines.

Problems that occurred after the administration of the Astrazeneca vaccine offer another rare example of researchers being able to definitively conclude early on that the vaccine itself was the source of problems.

"It happened rarely, but it was clear the vaccine had caused a very specific pattern of cerebral vein thrombosis that is otherwise unknown, because it was associated with a very specific type of antibody that developed in response to a cross-reaction with the vaccine," Prüss says.

All of this is complicated by the fact that people across Germany (and the world) experience headaches, chronic fatigue syndrome and cardiovascular disease on a daily basis — and not just since the pandemic and the major vaccination campaigns.

"Every day, 30 new multiple sclerosis diagnoses are made in Germany alone," says Prüss. Attributing all of these things to vaccination without hard evidence would do little to help people. An incorrect diagnosis of post-COVID vaccination syndrome could even hinder efforts to get patients with other diseases the therapy they actually need.

Additionally, one of the only ways to say with any degree of certainty that a person is suffering from post-COVID vaccination syndrome rather than long COVID is if that person started experiencing symptoms shortly — weeks, not months — after getting the jab and had not been diagnosed with COVID at any point before. This can be difficult, because people may have had a COVID infection yet not known it.

Prüss and others who deal with the topic suspect that a real post-COVID vaccination syndrome is probably very rare. The clinical pictures with which those affected come to the Charité consultation generally have other causes.

What happens in the bodies of post-COVID vaccination patients?

There is currently little conclusive medical understanding of what exactly is happening when patients develop post-COVID vaccination syndrome, but there are some theories.

Dr. Christine Falk, president of the German Society of Immunology, says the link between long COVID and post-COVID vaccination syndrome could be a cross-reaction with the spike protein — which is created by a vaccine or is an element of it — and the infection itself.

There are some people who, after being infected or vaccinated, not only create normal antibodies against the spike protein, but also experience a sort of cross reaction that produces antibodies that are inadvertently able to recognize endogenous structures — i.e., essential structures created by the body itself. These are called autoantibodies and are present in many people with autoimmune diseases.

That could be why post-COVID vaccination syndrome and long COVID have such similar symptoms.

With all of that said, Falk's idea is only a theory at this point, with much more research needed on the topic.

Edited by: Carla Bleiker, Anna Sacco

This article was originally published on June 13, 2023. It was updated on July 6, 2023, to include coverage of a public statement from the International Coalition of Medicines Regulatory Authorities (ICMRA) "on the safety of COVID-19 vaccines."