United Airlines is the latest company to offer vaccine incentives. The airline said vaccinated customers have the chance to win a year's worth of travel.Through June 22, any member of United's MileagePlus loyalty program can upload proof of vaccination on the company's website or app. Thirty people will win roundtrip flights for two to anywhere in the world, and five people can win the grand prize of free travel for a whole year.Other companies are giving vaccine incentives that include free doughnuts, beer and gift cards to get more people vaccinated.Some states, including Maryland, are trying to improve their vaccination rate by giving away millions of dollars in a special lottery.