England’s Premier League offers to ‘reward’ soccer teams that increase vaccine compliance.

Concerned that a significant number of its players remained unvaccinated, the Premier League, the top league in English soccer, is offering its 20 clubs an unspecified “reward” for getting more of their rosters immunized.

The proposal was set out in a letter sent to the teams last week, which said only seven of the 20 teams had more than half their players fully vaccinated. The figure has alarmed officials of a league that features players from all over the world who regularly cross national borders for competitions.

“It is increasingly clear that full vaccination will be the key criteria for government and health authorities, in terms of international travel and potential Covid certification at large-scale events,” the letter said.

The efforts of the Premier League, the world’s most popular domestic soccer league, to push for greater vaccine uptake mirror the challenges faced by other sports leagues, including the National Basketball Association in the United States. The N.B.A. players’ union has strongly opposed mandatory vaccinations, and unvaccinated players must submit to daily testing.

The Premier League’s letter did not outline what type of reward teams with high vaccination rates could expect. Officials briefed on the matter said the rewards were likely to be a loosening of the stringent coronavirus protocols in place since the league resumed play last year after being suspended because of the pandemic.

“We are considering if and how best we can ‘reward’ those squads/players who are most Covid-compliant,” the league told the clubs.

Many Premier League players also compete on national teams, whose next round of games begins in October. Some league clubs have refused to release players selected by national teams in South America, despite the threat of penalties from the sport’s governing body, because of concerns that when the players returned to Britain, they would be subjected to quarantine, as other travelers are when arriving from that continent.

Premier League teams are tested regularly, and coronavirus cases have continued to occur. N’Golo Kante, a star midfielder for Chelsea who is also on the French national team, tested positive for the virus this week, ruling him out from playing in Chelsea’s Champions League match with Juventus, an Italian team, on Wednesday, as well as an upcoming league game in Britain and a French national team match next week.

Thomas Tuchel, coach of the Chelsea club, told reporters on Tuesday that he did not know how many of the players on his roster were vaccinated.

“We are a reflection of society, the players are adults and they have a free choice,” he said, adding, “I know the situation is far from over. It makes you very aware it is not over.”