DENVER (AP) — Health officials in Colorado say they never received one of two deliveries sent as part of an effort to test the nation's vaccine distribution system because of a mistake on a mailing label, offering a glimpse into what could go wrong when the actual vaccine for the coronavirus is distributed in the coming weeks and months.

In November, the Centers for Disease Control and Prevention chose Colorado and nine other jurisdictions across the country to participate in a pilot run of an “end-to-end logistics readiness test” for the distribution of a COVID-19 vaccine.

State health officials in Colorado initially announced Nov. 25 they had received a thermal shipper designed to contain the vaccine, as well as a “mock” ancillary kit, which typically includes supplies such as syringes, needles, alcohol prep pads, surgical masks, face shields and vaccination cards.

But the Colorado Department of Public Health and Environment released a statement to The Associated Press on Wednesday night saying the state received the thermal shipper, but the ancillary kit was accidentally sent to Kentucky.

“These materials were not critical for the test and did not cause any problems,” according to the statement. “The state completed its portion of the simulation successfully and learned along the way. Situations like this allow the state to be prepared for ‘real life scenarios’ that we might face as the vaccine is distributed in order to effectively deal with them.”

To successfully complete the test, the state was expected to identify a location to receive the shipment, use the CDC’s Vaccine Tracking System with a simulated national drug code and report that it had received the thermal shipper and “mock” ancillary kits.

The state health department did not say which participant in the program sent the supplies to the wrong state and did not respond to a request for a phone interview. Meanwhile, communications between The Associated Press and the four companies involved in the pilot program — FedEx, UPS, Pfizer and the pharmaceutical supplier McKesson — did not clear up who made the mistake, and the CDC did not respond to an emailed request for comment.

Regardless of which company sent the shipment to the wrong place, the error underscores the importance of testing logistical systems before the real vaccine is distributed. It also shows that something as seemingly harmless as a mislabeled address could be the difference between life and death among those who are more vulnerable to COVID-19, including nursing home residents who will be among the first to receive the vaccine.

Colorado has been hit with a substantial spike in COVID-19 cases, with one in 40 residents believed to be contagious. More than 241,000 people in the state have tested positive, and more than 2,700 have died from the virus since it started its rapid spread in the spring, according to state health officials.

For most people, the coronavirus causes mild or moderate symptoms, such as fever and cough that clear up in two to three weeks. For some — especially older adults and people with existing health problems — it can cause more severe illness, including pneumonia, and death.