Moderna Inc. on Wednesday reported $6 billion in first-quarter revenue from its COVID-19 vaccine that breezed past estimates, and said it expects vaccine sales to be higher in the second half of the year than in the first.

The company's shares rose 6.8% to $156.46 before the opening bell. The results eased some worries about vaccine sales this year as an increasing number of people globally receive a complete vaccination course.

Still, future sales remain uncertain as countries relax pandemic curbs, cases fall in some regions and the need for annual booster shots remains unclear.

Moderna, like rival Pfizer, maintained its full-year sales forecast for the vaccine at $21 billion. The bigger drugmaker also stuck to its forecast for its COVID pill, signaling a dearth of new sales contracts for the drug.

Moderna's vaccine is based on messenger RNA (mRNA), the same technology that is also used by Pfizer, and recently gained authorization for a second booster dose in the United States.

U.S. regulators granted authorization for boosters of both vaccines in March for people age 50 and older, citing data showing waning immunity and the risks posed by Omicron variants of the virus.

Moderna Chief Executive Officer Stephane Bancel told CNBC that he hoped people, especially those at high-risk of disease, would get a booster dose in the fall.

Moderna's COVID shot has propelled it from a developer of experimental vaccine technology with no approved products to a vaccine maker with billions of dollars in sales.

Sales of its vaccine rose to $5.9 billion in the first quarter from $1.7 billion a year ago. Total sales of $6.1 billion beat estimates of $4.62 billion, according to IBES data from Refinitiv.

The company said it expected to test a booster shot to target the Omicron variant, in addition to the original coronavirus, in a late-stage trial in the second quarter.