An already beleaguered U.S. vaccine roll-out has hit a new snag: Thousands of doses of the COVID-19 vaccine around the country are being dumped after being stored at the wrong temperature.

More than 4,000 doses of the Moderna vaccine were wasted in Maine on Tuesday, when clinics opened their deliveries to find a red X on each of the vials, signifying that the vaccine had reached too high a temperature to be effective, according to the Portland Press Herald. That same day, the Detroit Free Press reported that providers in Michigan had the opposite problem: Nearly 12,000 doses were tossed because the vaccine had become too cold.

Just Wednesday, the Ohio Department of Health suspended a provider from its vaccination program after the company let nearly 900 doses exceed the recommended temperature range. And hundreds of doses were ruined earlier this month in Colorado and Louisiana, after power outages in both states caused the storage refrigerators to shut down.

Moderna vaccines must be kept in the highly specific temperature range of minus-13 to 5 degrees Fahrenheit. The doses are shipped to distributors frozen, and can be kept at that temperature for up to six months, or in a refrigerator for up to 30 days. The Pfizer vaccine, which has not started shipping, requires even colder storage temperatures.

In Michigan and Maine, state officials emphasized that the incidents signified the system was working: The ruined vaccines were quickly identified and not distributed to any patients. But they were also a significant hiccup in an already behind-schedule vaccine rollout that has been plagued with slowdowns and slip-ups.

Approximately 16.5 million people in the U.S. have been vaccinated to date—well short of the 20 million that Health and Human Services Secretary Alex Azar predicted would be vaccinated by the end of 2020. In some cases, staffing shortages have slowed distribution; in others, doses have been purposefully destroyed. Several states have had to change who is eligible for the vaccine after receiving more doses than they were able to distribute.

The malfunction in Maine, which appeared to have happened during packing or shipping, caused 35 of the 50 vaccine centers expecting shipments Monday to receive unusable doses, according to Dr. Nirav Shah, director of the Maine Center for Disease Control and Prevention. Replacement doses were set to arrive Tuesday or Wednesday.

The 21 shipments in Michigan were also spoiled in transit, according to state health officials. Distribution company McKesson Corp. has already shipped replacements for the majority of the doses and is investigating what went wrong.

The doses in Ohio were being held by health-care company SpecialtyRx for distribution at long-term health facilities when employees learned that 890 of them had been stored at the incorrect temperature. The company has since been banned from Ohio’s vaccine distribution program, and the incident is under investigation by the Ohio State Board of Pharmacy.

Two separate facilities in Colorado lost more than 100 doses each in one week last month—once, when a portable storage unit in Pueblo malfunctioned, and the next day, when a power outage in Lakewood caused a refrigerator to crash. A week earlier, Louisiana also lost 120 doses of the vaccine when a storm caused a distribution facility in Baton Rouge to lose power.

“A little bit of me dies every time we hear about a dose that’s been lost,” Louisiana Assistant State Health Officer Joseph Kanter said at a press conference. “We know how important these doses are, and it really does get at me.”