Just before 9:30 a.m. on Monday, Sandra Lindsay, an intensive care nurse at Long Island Jewish Medical Center in Queens, New York, became one of the first people to get vaccinated against COVID-19.

The jab was the highly anticipated start of what public health experts hope is a nationwide wave of vaccinations that will signal the beginning of the end of the pandemic. The news coincided with a dark new milestone for the country—more than 300,000 confirmed COVID-19 deaths.

Only one vaccine, from Pfizer-BioNTech, has received emergency use authorization from the Food and Drug Administration to begin shipping. Another vaccine, from Massachusetts-based biotech Moderna, is up for review later this week.

Read more: More Than 300,000 People in the U.S. Have Died From COVID-19

Many hospitals turned the first jabs into a media opportunity, documenting on video the otherwise routine vaccination. But, the seemingly simple process belied a herculean logistical effort to make it happen—one that may still have kinks to be worked out over the coming days and weeks as more doses are delivered and people start receiving the second of the two-dose regimen about a month later.

First, the limited number of doses means that there won’t be enough to vaccinate even the first priority groups of healthcare workers and people living in long term care facilities. States asked hospitals, health facilities and providers to submit requests for doses, and allocated shots based on factors including the most urgent need, the ability to store the vaccines properly and the ability to use all of the first doses quickly. The Pfizer-BioNTech vaccine relies on a new technology based on mRNA, which needs to be stored frozen at ultra-cold temperatures that only specially designed freezers can reach, so facilities that have these freezers are more likely to receive the vaccine first.