Steven Brandenburg, 46, is accused of removing from cold storage 57 vials with 570 doses of the Moderna vaccine in Grafton, Wisconsin, in December.

Without refrigeration, the vaccine becomes ineffective.

Mr Brandenburg could be jailed for 20 years on two charges of attempting to tamper with consumer products.

He also faces a $250,000 (£182,000) fine.

In a statement, the attorney’s office in Wisconsin said Mr Brandenburg “signed a plea agreement acknowledging that he was guilty of attempting to render hundreds of doses of Covid-19 vaccine ineffective”.

He was charged “with two counts of attempting to tamper with consumer products, with reckless disregard for the risk that another person will be placed in danger of death or bodily injury”.

The pharmacist admitted that he “believed in various ‘conspiracy theories’; subscribed to notions of “alternative history”; and was sceptical of vaccines in general and the Moderna vaccine specifically”, the statement added.

Mr Brandenburg allegedly tampered with the vials on two successive night shifts at the pharmacy in late December. Each time, he left the vials out of the refrigerator for several hours before putting them back.

The attorney’s office said 57 people had received the jabs from the vials before the tampering was uncovered.

“Pharmacists rank among some of the most trusted professionals,” said FBI Special Agent in Charge Robert Hughes. “This individual used his special access to tamper with vials of the much needed Covid-19 vaccine.”