"We are cognizant that in this type of environment - fueled by the ease and convenience of e-commerce and anonymity afforded by the Internet - there will be an increase in the prevalence of fraud, counterfeit and other illicit activity as it relates to vaccines and treatments for COVID-19," Pfizer said in a statement.
A source tells ABC News a substance inside the vials of fake vaccine in Poland was a cosmetic product. Pfizer is working with local authorities in the matter.
Mexican authorities said Wednesday that fake coronavirus vaccines represent "a risk to health."
In February, the health secretary of Mexico's northern border state of Nuevo Leon warned about "clandestine" sale of "alleged Covid vaccine."
An investigation found at least 80 or 90 people were getting vaccine for the equivalent of hundreds of dollars per dose, but the substance was unknown, and may have been water, or something even more harmful, Manuel de la O Cavazos, the health secretary of Nuevo León, said at the time.
The Mexican government urged people not to try to get vaccines from online or private sellers, because all real vaccines are being distributed by the government.
However, there haven't been enough doses to go around, prompting some Mexicans to fall prey to fake vaccines, or travel to the United States to get vaccinated.
Pfizer is warning the public not to trust online vendors selling the vaccine.
"Patients should never try to secure a vaccine online - no legitimate vaccine is sold online - and only get vaccinated at official vaccination centers or by certified healthcare providers," the spokesperson told ABC News.
This latest volley of fraud was first reported by The Wall Street Journal, however, it's not the first time this issue has been raised on the international stage, and adds to the running tally of COVID-19 scam attempts.
ABC News and the Associated Press contributed to this report.
The video in the media player above is from a previous report.