Visitors will not be allowed into Guernsey's Princess Elizabeth Hospital (PEH) from Tuesday, the States has announced.
The Committee for Health and Social Care (HSC) said the decision was made with "regret" after requests to limit visitor numbers were not effective.
Exceptions will be made for visitors of people in end-of-life care and patients on maternity and children's wards.
Those visits can be arranged by contacting ward managers, HSC said.
Alderney's Mignot Memorial Hospital also announced it would close to visitors.
HSC, alongside the Medical Specialist Group, announced the suspension of all "non-urgent" outpatient appointments from Monday.
These include appointments at the PEH, Alexandra House and Mill House.
The committee said some appointments would be conducted by telephone where "feasible" and all patients would be contacted about the decision.
HSC also outlined measures it was putting in place to increase "resilience" of the hospital and critical care, including:
- Changes to ward layout at the hospital to create a special critical care area
- Beginning the process of appointing extra temporary agency nurses
- A dedicated team to ensure the supply of personal protective equipment for healthcare staff
HSC president Heidi Soulsby asked for "support and understanding" from islanders over the changes.
Deputy Soulsby stressed all 20 people who had positive for the virus "had a clear travel history" and the committee would "continue to assess and respond to the rapidly changing" situation.
She said: "These changes will continue to ensure our services can be maintained for those who most need our care.
"We are facing extraordinary circumstances currently, and I thank all our staff within HSC and across our partners for their continued focus and dedication to our community".