COVID-19 is threatening to wreck the Premier League schedule. There were three matches postponed in the English top flight in December and one in January, the first since "Project Restart" saw football return in June, with further matches threatened due to a spike in positive cases and number of stars in self-isolation.

The spate of positive COVID-19 results -- the last round of testing on Dec. 29 returned 18 positives from the 1,479 taken, which was the most this season -- in the Premier League, and the growing uncertainty over when and if matches could be played led to calls for a two-week break that would allow teams to regroup. Throughout the coronavirus pandemic, the Premier League has remained steadfast in its trust in the protocols and faith in keeping the league going.

With the FA Cup third round across the weekend of Jan. 8-11 and a hectic, unrelenting schedule set to run through May 23, there are growing fears over whether football in England is destined for an unsustainable fixture pile-up during a pandemic. Here's the state of play.

How did we get to this point?

There have been sporadic individual positive COVID-19 results in the Premier League since March. The Premier League tracks them each week and publishes the results, but as the UK battles a rapid increase in numbers -- Dec. 29 saw a record 53,135 new cases across the UK -- and a new, whole-country lockdown, football has been hit hard a second time, with matches at all levels postponed and training grounds shut to prevent further spread.

When a team calls for a match to be delayed, they have to meet certain criteria, with the decision ultimately down to the Premier League board (Premier League CEO Richard Masters, chair Gary Hoffman, Claudia Arney and Kevin Beeston.) The board also consults Premier League medical advisor Dr. Mark Gillett and Public Health England (PHE). There are a number of mitigating factors, with one source telling ESPN that the decision-making criteria is not clear cut. The Premier League guidebook says that "permission will not be granted to postpone a League match where the applicant club has 14 or more players listed on its squad list available," with everything judged on a "case-by-case basis."

Newcastle United's match against Aston Villa, scheduled for Dec. 4, was the first Premier League game to be postponed due to COVID-19. The board deemed Steve Bruce's team unable "to train and adequately and safely prepare for their match." Four weeks on, Newcastle still had players out with long-lasting symptoms.

It was different when Manchester City contacted the Premier League on Dec. 28, asking for their match at Everton later that day to be postponed. On Christmas Day, three days after they beat Arsenal 4-1 in the Carabao Cup quarterfinal, City announced that Gabriel Jesus and Kyle Walker and two staff members had tested positive. On Dec. 28 they had another three players test positive. The Premier League board agreed to Man City's request due to "uncertainty" over how far the virus had spread in the squad. They would go on to play Chelsea on Jan. 3 and beat them 3-1 despite having players absent due to self-isolating. City defender Eric Garcia later confirmed he too the virus, while City now have four players from their women's team in isolation.

Fulham's matches against Tottenham Hotspur (Dec. 30) and Burnley (Jan. 3) were also called off due to an outbreak at the club with a "significant number" of positive tests. A Premier League statement read: "Following Fulham's request to rearrange the Burnley fixture, the Premier League's board has taken into consideration further independent medical advice and decided to postpone the match due to the exceptional circumstances of the ongoing outbreak at the club."

"Long COVID" and fixture congestion: Why this could get worse

With the 2020-21 Premier League season already starting a month late due to the delayed end to 2019-20, the rescheduled Euro 2020 tournament (set for the summer of 2021) means that domestic seasons can finish no later than May 23. That leaves 32 weekends, five midweeks and one bank holiday to fit all the Premier League fixtures in, a loss of two weekend matchdates compared to a regular season. Despite the winter break being scrapped, alongside FA Cup replays, only three fixture catch-up dates remain in the calendar for those still playing in the Champions League and Europa League.

Man City have to re-arrange two matches: their season opener against Aston Villa was postponed given the late finish of the previous European campaign, and will now be played on Jan. 20. They also have to find a new date for the game against Everton. Add in the complexities of juggling Premier League commitments with their Champions League, FA Cup and Carabao Cup campaigns, and further hits to City's schedule due to COVID-19 could trigger an unsustainable fixture pile-up later in the season.

Further down the England football pyramid, the virus was wreaking havoc in the fixture schedule. Seven of the 12 planned matches in League 1 on Dec. 29 were called off. Sunderland had to postpone four consecutive games after 13 players tested positive and from the Championship to the National League, a total of 24 games were postponed in December. The English Football League began advising clubs against training indoors or using indoor gyms, sources told ESPN.

Premier League managers were rotating squads to cope with the hectic festive period, while Liverpool appointed a new head of recovery and performance to aid their scheduling headache and help battle their lengthy injury list. Manager Jurgen Klopp was vocal in his support for Premier League clubs being able to use five substitutes to help avoid burnout, but while the league's board approved clubs being able to name nine replacements (ahead of the introduction of concussion subs later this month), they stuck to their preseason plan of only permitting managers to use the conventional three. It caused concern outside of the Premier League with England boss Gareth Southgate saying: "I think from a national team point of view, a compact season like this, it's always a concern what you're going to get at the end of it."

Then there are the long-term health concerns. To stem the outbreak in December, Newcastle shut their Darsley Park training ground on the advice of PHE. "When you see the numbers [positive tests] in the Premier League, you realise the protocols they have put in place have worked," Bruce said. "However, when it does strike, you realise how quickly and easily it spreads. The quickness of it was scary. The decision to shut down the training ground, even though it was the last thing I wanted to be doing, that proved pivotal. Within 10 days, we didn't have anyone test positive."

Footballers are not immune from the long-lasting effects of the virus. Both Bruce's club captain, Jamaal Lascelles, and star player, Allan Saint-Maximin, were both sidelined for several weeks after contracting the virus.

"The big thing is fatigue and the way it has left you feeling tired," Bruce said. "The players are fit, healthy and young, and for the vast majority, it washes over them. But unfortunately, for two of the players, in particular, it has not washed over them.

"If you ever underestimated this thing, then don't. You're talking about elite professionals here, and the way it got hold of everybody was quite scary stuff."

Isaac Hayden, the Newcastle midfielder, spoke of how he had "underestimated" the power of the virus. "I was in bed for 10 days and couldn't do anything," he said. Chelsea's Kai Havertz was another who took three or so weeks to recover.

One doctor who's worked in football told ESPN they were "stunned" when they heard about players suffering with "Long COVID."

"It came as a huge surprise," the doctor said. "None of the players I dealt with had to deal long-term effects or symptoms. These players are in incredible health, but I wonder now if it's more prevalent than we might think. It's a concern as we don't know that much about it."

The mental side of the relentless fixture schedule and COVID-19-related stress has also impacted players. One former club staff member told ESPN of how the players are struggling with mental burnout. Medical professionals, meanwhile, are most concerned about the employees at a club who are older and at a higher risk of severe complications and illness.

"More thought needs to be given to the people behind the scenes," one doctor told ESPN. "Players by and large will be okay, and most will be asymptomatic, but then you have those who have been at the club for 40 years washing kits and they're at risk.

"It won't be your superstar [in danger] -- it'll be the person who has worked at the club for 30 years, on a small salary, who has loved their job but will get it off one of the players and they'll have issues. The human cost needs to be what we are mindful of. The availability of players shouldn't really be what we're all most worried about. We need to look beyond the lines and the risk to those you'll never hear about."

How can football move forward?

For a start, the Premier League has confirmed they are going to test players more vigorously. When "Project Restart" kicked into gear, teams were tested twice a week, which was then reduced to once a week prior to the start of the 2020-21 season. Teams were gradually tested more throughout December and will be tested twice a week from the start of January.

There have been growing calls in some circles for a "circuit break" -- or mid-season two-week "rest period" -- with West Brom's new manager Sam Allardyce supporting such an idea.

"Everyone's safety is more important than anything else," West Brom boss Sam Allardyce said recently. "I'm 66 years old and the last thing I want to do is catch COVID. I'm very concerned for myself and football in general. If that's what needs to be done, we need to do it."

Manchester United boss Ole Gunnar Solskjaer was less keen to see the season halted. "I can't see the benefit in having a 'circuit break,' whatever it's going to be called," Solskjaer said after his United side beat Wolves on Dec. 29. "Because when are we going to play the games? We all know this year is so difficult, but I don't think stopping the games is going to make a big, big change."

Sources told ESPN that, at this stage, the Premier League is not considering a circuit break as a realistic option. Jack Grealish, the Aston Villa captain, showed his support for Gary Neville's tweet where he disagreed with the logic of pausing the schedule.

But unless the government orders a shutdown of professional sport, it looks unlikely at this stage anything will get in the way of the Premier League continuing to play games. Adding to the confusion is the lack of a contingency plan if COVID-19 cases rise to the extent the season needs to be curtailed, a situation described as "embarrassing" by a source at one top-flight club. One source told ESPN they believed the only way the Premier League would call for a hiatus would be if the positive results grew into the hundreds. The doctor added: "We shouldn't be carrying on, mid-pandemic, just because the fixtures are hard to re-arrange, or because of the financial implications.

"People's lives are at risk. With all of this, we need to ask why this sport is continuing. Small businesses are suffering badly, and I think football is at a moral crossroads." Sources told ESPN the Premier League would not seek a private source for COVID-19 vaccinations so that its players could be immunized and continue the season safely.

In the meantime, clubs face increased scrutiny over their respective COVID-19 protocols and additional rounds of testing as the Premier League tries to keep the show on the road. Sources have told ESPN there is a growing unrest and confusion among the clubs over the abstract nature of these criteria. Sources say some clubs are unsure exactly why some matches are permitted to go ahead, and others are postponed. The timing also frustrated clubs, with some games called off just hours before kick-off.

Arsenal boss Mikel Arteta tested positive for COVID-19 before the league was halted in March, but nine months on, it remains unclear what it'll take this time around.

"We're all concerned with our own health and what's going on around us," Arteta said ahead of his team's first match of 2021. "All the protocols we have in place, everything we do around the training ground and playing outdoors minimises the risk a lot.

"As long as we can, we have to carry on doing that -- obviously without putting anybody at risk. But we've shown the system is working. Okay in the last week or so something has happened [but] we'll have more restrictions, more tests to be as efficient as we were before, and we'll see. But I think it can work and we can carry on."