Republic of Ireland boss Stephen Kenny says he is trying to persuade some of his players to receive Covid-19 vaccinations, but insists "you have to respect individual wishes".
Kenny was speaking after naming his 25-man squad for September's crucial World Cup qualifying triple-header.
The Republic face Portugal away on Wednesday before hosting Azerbaijan and Serbia on 4 and 7 September.
"A number of players have had vaccines and some haven't," said Kenny.
"Some have had one. In England, the AstraZeneca vaccine has quite a long period between first and second, so a couple of players have been caught with the first one, and some players have decided not to get it, so that's where it is."
Kenny's reign has been heavily disrupted by Covid issues - forwards Aaron Connolly and Adam Idah were both ruled out of October's Euro 2020 play-off semi-final against Slovakia as close contacts of a positive case.
In-form West Brom striker Callum Robinson, 26, has now been ruled out of the upcoming Portugal game after testing positive for the second time, having also returned a positive test in November.
Preston midfielder Alan Browne, who also tested positive in November, misses out after the 26-year-old came into close contact with a positive case.
Kenny is hopeful that both Browne and Robinson can link up with the squad later in the international window, although the former is also nursing a muscle problem.
UK-based players would no longer have to isolate as close contacts of a third party who had tested positive if they had been double-jabbed.
"Yes, and that has happened with a player, for example, who had one vaccine and hadn't had the second and wouldn't have had to isolate," said Kenny when asked if he was pushing his players to opt for vaccination.
"But that's the situation we're in. Ideally we'd want everyone vaccinated, but the players are men as individuals and they have the right to make their own decisions, and we have to respect that.
"In an ideal world, we would prefer to have everyone vaccinated and that wouldn't be an issue, but you have to respect the individual wishes."
Kenny is without several players for September's qualifiers because of injury with the Republic in need of points after starting their campaign with defeats by Serbia and Luxembourg.
While Burnley defender Nathan Collins received his first call-up, Kenny is without Enda Stevens, Callum O'Dowda, Jason Knight and Chiedozie Ogbene.
There was also no place in the squad for experienced internationals Ciaran Clark, James McCarthy, Robbie Brady and Darren Randolph, but Kenny has said the door is "absolutely not" closed for the West Ham goalkeeper after opting for Caoimhin Kelleher, Gavin Bazunu and Mark Travers as his three stoppers.
"Darren has been a terrific goalkeeper for Ireland," said Kenny, who is still looking for his first competitive win as Republic boss.
"Obviously he's had good pre-season then he got Covid, which interrupted his training. He's back training now, but the other three goalkeepers have all been excellent in their games recently.
"I am very happy with their form and they stay in the squad."
St Mirren's Jamie McGrath and Norwich defender Andrew Omobamidele retain their place in the squad having been called up in June, while Brighton forward Aaron Connolly and Newcastle midfielder Jeff Hendrick return after missing the summer friendlies.
The Republic squad fly out to Portugal on Sunday before Wednesday's encounter with group favourites Portugal, who have included Cristiano Ronaldo in their squad.
The 36-year-old forward had been a doubt after injuring his arm in Juventus training earlier this week.
Ronaldo will become the world's all-time leading international goalscorer if he finds the back of the net against the Irish with the five-time Ballon d'Or winner currently tied on 109 with former Iran striker Ali Daei.
Republic of Ireland squad
Goalkeepers: Caoimhin Kelleher (Liverpool), Gavin Bazunu (Portsmouth, on loan from Manchester City), Mark Travers (AFC Bournemouth)
Defenders: Seamus Coleman (Everton), Matt Doherty (Tottenham Hotspur), Shane Duffy (Brighton and Hove Albion), John Egan (Sheffield United), Dara O'Shea (West Bromwich Albion), Ryan Manning (Swansea City), James McClean (Wigan Athletic), Andrew Omobamidele (Norwich City), Nathan Collins (Burnley).
Midfielders: Josh Cullen (Anderlecht), Jeff Hendrick (Newcastle United), Conor Hourihane (Aston Villa), Harry Arter (Nottingham Forest), Jamie McGrath (St. Mirren), Jayson Molumby (Brighton and Hove Albion)
Attackers: Aaron Connolly (Brighton and Hove Albion), James Collins (Cardiff City), Daryl Horgan (Wycombe Wanderers), Adam Idah (Norwich City), Troy Parrott (MK Dons, on loan from Tottenham Hotspur), Shane Long (Southampton), Ronan Curtis (Portsmouth).