Neurological adverse events are very rare after COVID-19 vaccination and less frequent than after SARS-CoV-2 infection

On 10 May 2023, a tweet from an account called Leading Report claimed that “COVID vaccines could be causing ‘long-term brain damage’”. The claim was based on a review published by Hosseini and Askari in the European Journal of Medical Research on 25 February 2023 that evaluated the potential effects of COVID-19 vaccines on the nervous system.

Although the tweet was deleted a few days later, it still gathered more than 2.4 million views and 2,800 retweets, including a particularly viral retweet by Twitter user Catturd2, who has 1.7 million followers. This retweet alone accumulated 1.8 million additional views and 9,000 retweets, spreading on Instagram as well.

Catturd2 became popular thanks to its frequent interactions with Twitter owner Elon Musk and former U.S. president Donald Trump and has propagated political and COVID-19 misinformation on multiple occasions. In addition, Catturd2 also participated in a networked harassment campaign targeting U.S. Representative Alexandria Ocasio-Cortez, according to an investigation by the digital research platform Media Manipulation Casebook.

The same review that the tweet cited was also picked up in a Substack article by Colleen Huber, a naturopath who promotes unproven cancer cures and regularly spreads COVID-19 misinformation. Huber’s article claimed that there were ways in which COVID-19 vaccines supposedly enter the brain and damage neurons, presenting Hosseini and Askari’s review as evidence of post-vaccination “neurological injuries”.

However, the review doesn’t provide such evidence, and the claim that COVID-19 vaccines cause or increase the risk of neurological problems based solely on this review is unsupported, as we will explain below.

The review by Hosseini and Askari didn’t show that COVID-19 caused neurological problems

In their review, Hosseini and Askari evaluated neurological events described in the scientific literature following vaccination with an mRNA (Moderna and Pfizer-BioNTech), viral vector (AstraZeneca, Johnson & Johnson, and Sputnik), or inactivated virus (Sinovac, Sinopharm, and Covaxin) vaccine between 2020 and February 2022.

Based on the results from these studies, the review concluded that the most common neurological adverse events reported following COVID-19 vaccination were mild and transient, such as headache, fatigue, and joint pain. Less common symptoms included tremors, tinnitus, and a reactivation of the virus that causes chickenpox in a person who had the disease, and that can cause nerve pain and a broad range of neurological complications. Finally, serious neurological adverse events included venous sinus thrombosis—a rare cause of brain stroke—transverse myelitis, Guillain-Barré syndrome, and multiple sclerosis.

Posts and articles showing this list of neurological conditions, along with claims of brain damage, can leave readers with the idea that the COVID-19 vaccines are unsafe and responsible for these conditions, even when this wasn’t explicitly stated. This notion that any medical problem occurring after vaccination is necessarily a side effect of vaccination is common but misleading.

There is a clear difference between an adverse event, which is any health problem that occurs following vaccination regardless of what caused it, and a side effect. Side effects refer exclusively to those health problems caused by the vaccine, such as pain and swelling at the injection site.

Many different factors, including vaccination but also infection, can trigger the neurological conditions listed above. Thus, while some of these adverse events might represent actual side effects of the vaccine, many others are unrelated to vaccination and caused instead by other factors.

Although the review by Hosseini and Askari mentioned “side effects” and “complications” on several occasions, many of the studies they evaluated are case reports and analyses of surveillance data, including data from the U.S. Vaccine Adverse Event Reporting System (VAERS). Such data refers to adverse events and are generally insufficient on their own to demonstrate a causal link between the adverse event and the vaccine, as the authors acknowledged in their conclusion:

“[D]iscovering whether these disorders are accidental or whether the vaccine is the main cause of them requires future studies, ongoing efforts to gather evidence, and long-term monitoring.”

To determine whether the COVID-19 vaccine caused these neurological events would require further investigation, such as evaluating whether these events occurred more frequently in vaccinated individuals compared to the overall population. However, Hosseini and Askari didn’t conduct such investigations and therefore, these results don’t support claims linking COVID-19 vaccines with neurological problems nor provide evidence that the vaccines are unsafe.

Neurological problems following COVID-19 vaccination are rare and much less likely than after SARS-CoV-2 infection

Although no vaccine is without side effects, the most common side effects of COVID-19 vaccination, such as headache, fatigue, and soreness at the injection site, are mild and resolve within a few days.

As the number of vaccinated people grew to millions, a few rare but serious adverse events linked to vaccination emerged, including heart inflammation, severe allergic reactions, and a rare blood clotting condition with low platelets.

In 2022, the U.S. Food and Drug Administration and the European Medicines Agency also listed Guillain-Barré syndrome, a condition that affects the nerves and causes pain, muscle weakness, and sometimes paralysis, as a possible rare side effect of viral vector vaccines. This decision followed several reports of people who developed this condition after receiving the vaccine from AstraZeneca and Johnson & Johnson. The agencies evaluated the rate of Guillain-Barré syndrome among vaccinated people, finding that it was slightly higher than expected for the general population.

Because Guillain-Barré syndrome had also been linked to other vaccines, like the shingles vaccine Shingrix and the influenza A (H1N1) vaccine^[1], the agencies considered there was “at least a reasonable possibility” of a causal link. However, there is no definite proof of such a causal link.

It is also important to keep in mind that Guillain-Barré syndrome happens frequently following an infection with a virus or a bacteria. This means that, even if a vaccine slightly increased the risk of developing this condition, it would still protect against the risk associated with infection.

While neurological adverse events occasionally occur following vaccination, current evidence is insufficient to establish whether COVID-19 vaccines caused these events. Furthermore, large clinical studies show that the risk of neurological problems following vaccination is extremely small, and lower than the risk after a SARS-CoV-2 infection.

For example, a U.K. study published in Nature Medicine in 2021 evaluated the rates of Guillain-Barré syndrome, Bell’s palsy, neurological disorders causing muscle weakness (myasthenic disorders), nerve and brain inflammation, brain hemorrhage, and stroke in 32.5 million adults in England and Scotland^[2].

Then, the researchers compared the rates of these conditions after receiving the Pfizer-BioNTech or the AstraZeneca COVID-19 vaccines to the background rates in the general population. People who received the Oxford-AstraZeneca COVID-19 vaccine showed 38 extra cases of Guillain-Barré syndrome per ten million doses. In addition, the study found an association between the Pfizer-BioNTech COVID-19 vaccine and a slight increase in the rate of stroke in England (60 extra cases per ten million doses) but not in Scotland.

But importantly, the study found that the highest risk of neurological problems was after SARS-CoV-2 infection. People who tested positive for COVID-19 had increased rates in three of the seven conditions analyzed, including brain inflammation, myasthenic disorders, and Guillain-Barré syndrome. Also, the rate of Guillain-Barré syndrome following infection was around four times higher than that observed after receiving the Oxford-AstraZeneca COVID-19 vaccine.

Another study published by Li et al. in BMJ in March 2022 arrived at similar conclusions after analyzing data from more than eight million people in the U.K. and Spain^[3]. This study found no difference between the rates of neurological problems in people who received an mRNA (Pfizer-BioNTech or Moderna) or a viral vector (Oxford-AstraZeneca or Johnson & Johnson) COVID-19 vaccine and the background rate in the general population. In contrast, people who had had a recent positive COVID-19 test showed increased rates of Bell’s palsy, encephalomyelitis, and Guillain-Barré syndrome.

Anton Pottegård and Olaf H Klungel, pharmacologists at the University of Southern Denmark and Utrech, respectively, explained in a BMJ editorial:

“We may never be able to tell exactly what caused an individual to develop a neurological condition, but covid-19 vaccination is a highly unlikely reason for most”.

Conclusion

The claim that a review by Hosseini and Askari showed COVID-19 vaccines could cause long-term brain damage is inaccurate and unsupported. This review only described the most common neurological adverse events following vaccination but didn’t demonstrate that the COVID-19 vaccines caused these events.

Available data from large epidemiological studies suggest that mRNA vaccines don’t increase the risk of neurological problems. While viral vector vaccines are associated with a slightly higher rate of Guillain-Barré syndrome compared to the general population, the overall risk is very small, and lower than the risk of developing neurological complications after a SARS-CoV-2 infection.

REFERENCES

• 1 – Salmon et al. (2013) Association between Guillain-Barré syndrome and influenza A (H1N1) 2009 monovalent inactivated vaccines in the USA: a meta-analysis. Lancet.
• 2 – Pantone et al. (2021) Neurological complications after first dose of COVID-19 vaccines and SARS-CoV-2 infection. Nature Medicine.
• 3 – Li et al. (2022) Association between covid-19 vaccination, SARS-CoV-2 infection, and risk of immune mediated neurological events: population based cohort and self-controlled case series analysis. BMJ.