The updated Covid-19 vaccine produced by US drugmaker Pfizer and German partner BioNTech may be linked to a specific type of brain stroke in older adults, according to preliminary research data being examined by US health authorities.

Health officials on Friday cited one of the many health databases of the U.S. Centers for Disease Control and Prevention (CDC), in pointing out that people 65 and older exhibited an increased risk of ischemic stroke 21 days after being given the Pfizer/BioNTech bivalent Covid-19 vaccine, compared with their risk on days 22-44.

Ischemic strokes are caused by blood clots which block blood pathways in the brain, denying parts of the brain oxygen and nutrients.

Authorities noted, the situation will require more investigation, and, “it is very unlikely that it represents a true clinical risk.”

Both BioNTech and Pfizer said in a statement they are aware of reports of ischemic strokes in individuals who were 65 and older who received the updated vaccine.

Pfizer added, “Neither Pfizer and BioNTech nor the CDC or the U.S. Food and Drug Administration (FDA) have observed similar findings across numerous other monitoring systems in the U.S. and globally and there is no evidence to conclude that ischemic stroke is associated with the use of the companies’ COVID-19 vaccines.”

Authorities noted the safety concern was not noted with Moderna’s bivalent vaccine, and they continue to recommend that everyone aged 6 months and older make sure they remain up to date on their immunizations.

The bivalent shots from both Moderna and Pfizer/BioNTech were engineered to target both the original strain of Covid-19 as well as the Omicron sub-variants. They are authorized for use in children 6 months and older.

If you or a loved one exhibit signs of a stroke, including sudden onset of physical weakness or paralysis on one side of the body, slurred speech, difficulty speaking, confusion, loss of balance, or other sudden cognitive changes, seek immediate medical attention.