The first COVID-19 vaccine developed by Pfizer (PFE) and BioNTech (BNTX) was shipped to sites across the U.S. over the weekend, launching the biggest mass vaccination campaign since polio.

The two-dose novel coronavirus vaccine, of which 2.9 million doses will be delivered in the first round to health workers and nursing home residents, is the first mRNA vaccine to be used on the market. The new technology requires significantly lower temperatures for storage, but its high efficacy rate has given hope that a second generation of vaccines using the same platform — and adjusting the cold storage needs— could set the trend for future vaccines.