(CNN)While the likes of the NFL are leading the way in public vaccine advocacy and transparency -- the league said it had reached a vaccination rate of over 90% among players by late September -- the uptake among footballers in the English Premier League remains shrouded in mystery.
With some 'more open to conspiracy theories,' vaccination rates of English Premier League players remain shrouded in mystery
The Premier League regularly releases details of how many players and club staff have been tested for Covid-19 and the number of positive tests, and some individual teams have disclosed vaccination rates, but the league declined CNN's requests to comment on the record for this story regarding vaccination data of players at its 20 clubs.
The BBC reported that in an email sent by the Premier League to its clubs in September the organization said: "Only seven clubs' squads are more than 50% fully vaccinated, so we have a way to go.
"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."
CNN has been unable to confirm the authenticity of the letter.
Currently, 85.4% of the UK population aged 12 and over has received at least one dose and 78.4% has received two doses.
"We are considering if and how best we can 'reward' those squads/players who are most Covid-compliant and who have opted to be vaccinated," the letter reportedly said.
The England national team is involved in a World Cup qualifying game against Andorra on Saturday and ahead of that match Tammy Abraham, who plays for Italian club Roma, became the first England player to confirm he had taken the Covid-19 vaccine.
England manager Gareth Southgate, speaking ahead of the Andorra game, said some of players were "more open to some of these conspiracy theories" surround Covid-19 vaccines "because they are reading social media more, they are perhaps more vulnerable to those sort of views."
"From what I can see there is a bit of confusion around. And there are several different threads there to why they are choosing to be jabbed or not to speak publicly about it," Southgate told reporters.
CNN has learnt that the 20 clubs provide the Premier League with their vaccine data on the condition it doesn't share it, and to help the league provide guidance on updated Covid restrictions at training facilities should clubs reach certain vaccination rates. The Premier League's stance is to encourage everyone to receive the vaccine.
Liverpool manager Jurgen Klopp recently told reporters that "99%" of his squad had been vaccinated and expressed frustration at the apparent slow uptake at rival teams, saying that refusing to take the vaccine "is a little bit like drink-driving."
"We all probably were in a situation where we had a beer or two and thought we still could drive but, [because of] the law, we are not allowed to drive so we don't drive," said Klopp.
"But this law is not there for protecting me when I drink two beers and want to drive, it's for protecting all the other people because I'm drunk and we accept that as a law.
"I don't take the vaccination only to protect me, I take the vaccination to protect all the people around me. I don't understand why that is a limitation of freedom because, if it is, then not being allowed to drink and drive is a limitation of freedom as well.
"I got the vaccination because I was concerned about myself but even more so about everybody around me. If I get [Covid] and I suffer from it -- my fault. If I get it and spread it to someone else -- my fault and not their fault."
Sky Sports News reported last week the Wolves squad was also fully vaccinated and Brentford and Leeds had more than 90% of their players fully vaccinated.
CNN contacted several leading Premier League clubs to ask about their current vaccine status, including Arsenal, Manchester City, Manchester United, Chelsea, Tottenham Hotspur and Leicester City.
Chelsea, Tottenham and Leicester declined to comment, while Arsenal, Manchester City and Manchester United hadn't responded at the time of publication.
In August, Southgate said he had received abuse for appearing in a video encouraging young people to get vaccinated.
"I'm not going to get too involved in this because I was asked to do a video supporting the vaccination programme, which I thought was responsible," said Southgate.
"And of all the things I received abuse for over the summer of which there's been several, that's probably the one I've received the most abuse over.
"I'm probably going to keep out of that argument for the time being."