Providers of the COVID-19 vaccine in Pennsylvania accidentally administered the Moderna follow-up vaccine injection as first doses, causing a shortage of booster shots, The Associated Press reported.

The mistake could mean that over 100,000 people will need to reschedule their appointments, Pennsylvania health officials said Wednesday, according to the AP.

Thirty thousand to 60,000 appointments for the second dose will need to be delayed by up to two weeks, said acting Pennsylvania Health Secretary Alison Beam. Delivery of 30,000 to 55,000 first doses will also need to be pushed back as officials try to get the state back on track, the AP reported. (RELATED: REPORT: Hundreds Of Doses Of Coronavirus Vaccine Were Thrown Away Due To Strict Rules, Mistakes)

The health department is still working to uncover the root cause of the problem, but says part of the issue resulted from shipments of the vaccine that were not properly labeled as first and second doses, the AP reported.

“When providers heard department staff saying things like, ‘There is no need for vaccine providers to hold back any first doses,’ they may have felt pressure to use all of the vaccine they had on hand, when in reality, some of those doses where earmarked for second dose vaccinations,” said Health Department spokesperson Barry Ciccocioppo, according to the AP.

Pennsylvania was given over 183,000 first doses this week, but shipments have been delayed due to severe weather, causing several clinics around the state to shut down temporarily.

Follow-up doses of the Moderna vaccine are usually given 28 days after the first dose is administered. However, the U.S. Centers for Disease Control and Prevention (CDC) recently adjusted this guideline and allows the shots to be given up to six weeks apart.