A lack of clarity from universities about how they will protect students who had to shield during lockdown "will pose further risk" to lives, the National Union of Students has warned.
It said a shortage of information about safe study, accommodation and support was "concerning" so close to term.
The government says universities should convey any plans "clearly" to students.
But some students fear they may have to shield in bedrooms and have called for more detail on safety measures.
Beth Bale has been shielding due to Crohn's Disease and a hormone deficiency and says she is concerned about the safety of accommodation and the possibility of a second wave.
"What if I get back to university and then four weeks in I'm asked to shield again and I can no longer access my kitchen or leave my room? My little box room will be all I have to ensure that I'm safe."
'Further risk to lives'
It is a fear echoed by many students among the 2.2 million people asked to shield by the UK government at the start of lockdown, which ended on 1 August.
But with just weeks before term begins, vulnerable students have said there has been little communication about what they can expect, in part, because universities themselves are grappling with an unprecedented situation.
The University of Surrey said it plans to "contact all new students" to see what support it can offer, and will try to meet specific accommodation requirements. But it said it doesn't actually have any plans in place yet.
The NUS said a "lack of clarity" from many universities about coronavirus measures was "concerning".
Sara Khan, who looks after student equality for the NUS, said: "Clear support pathways must be outlined for students moving into accommodation safely and accessing mental health services physically or remotely.
"A failure to put these plans into action will pose further risk to the lives of students, particularly shielding students."
The lack of specific information is something that worries Jennifer Geminiani who has been shielding for five months due to having the blood disorder Thalassemia, which can lead to complications in the body's organs.
She is about to start a master's degree in terrorism and politics at the University of St Andrews.
Rather than risk living in university accommodation she has decided to pay more for private housing, which she says gives her "security and certainty".
"I'm hoping that the student body will receive regular Covid-19 tests so that people can go safely and without worry to university."
She said her university life was already impacted by coronavirus before the summer break.
"I worked twice as hard as I used too, because I couldn't see my lecturers. I think what impacted me most was also not being able to see my friends. I love their presence, their ideas and thoughts."
Like Jennifer, Emma Beeden, a student at the University of Sussex, wants tangible safety measures put in place. She shielded following a kidney transplant.
"I am hoping the university as a whole will enforce mask-wearing as I know this is something that will make myself and others feel safer. I am sure the university has put lots of things in place, but it would be nice to know exactly what's going on."
The University of Sussex said its student support team would work with students who have long-term health conditions and those "anxious about returning to campus" to find "reasonable adjustments" for them.
But for those students who are on work placements as part of their degree, the worry around safety is just as acute.
Lauren Bradfield has Behçet's disease, which causes inflammation of the blood vessels and tissues.
As a second year student of paramedic sciences at the University of Surrey her hospital placement was cancelled at the start of lockdown to protect her health, but she is worried this will impact the completion of her degree.
"I am having to choose between my health and my career which is an awful decision to have to make. It feels almost impossible to plan going forward."
With just a few weeks to go before students leave the safety of their family homes there still remains a lot of uncertainty.
The Department for Education said universities are "autonomous institutions and we expect them to make judgements based on the latest public health guidance and communicate these clearly to students."
It said this includes carrying out risk assessments and it had "already seen a host of innovative measures being adopted, such as limiting travel times and student number rotas," although it did not say where these had been implemented.
While students anxiously await information that will ensure their safety, what has become clear is that post-lockdown learning is about far more than fine-tuning online lectures, it has also become a key lesson in health management.