By Michael McHugh and Q Radio News
Many thousands of people may already have coronavirus in Northern Ireland, the chief medical officer warned.
Testing will increase to more than 1,000 a day by next week - and that will help give a better idea of the infection's spread, Dr Michael McBride added.
Around 40,000 of the most vulnerable are to be written to by their GPs, urging them to be extra careful and stay at home.
Dr McBride said: "At this point in time, I suspect there are many thousands of individual cases in Northern Ireland."
He reiterated the importance of social distancing and good hand hygiene.
Three deaths linked to the virus have been recorded since Tuesday, the Public Health Agency (PHA) said. Another 37 people tested positive.
It brings to seven the total number of deaths from the outbreak.
Some workers at the Moy Park poultry producer in Northern Ireland walked out for a period on Wednesday because they hold safety concerns linked to Covid-19 and social distancing.
The company said it was taking their welfare seriously.
Stormont's first minister Arlene Foster and deputy first minister Michelle O'Neill called on non-essential employers to close - and if they had to remain open, to take steps to ensure staff safety.
Mrs O'Neill said she had seen vans full of men going to work on Wednesday.
She said: "That is not acceptable, that is not safe so that needs to be stopped."
Mrs Foster said those working in food production were extremely valued and urged their employers to look after them.
She added: "We need them to make sure that their employees are safe, not only safe but that they feel valued.
"We want them to continue to be able to provide food in this very difficult time."
Health Minister Robin Swann predicted there would be many more deaths.
He said: "That is the reality that is haunting us all during every waking hour, but we can reduce that death toll."
A pastor who contracted coronavirus said he felt like he was taking his last breath.
Mark McClurg, from Co Down, has been in the intensive care unit at the Ulster Hospital near Belfast for a week, but will soon be moving back to a ward.
He told the PA news agency: "It just feels like you are taking your last breath, the pain that you have all around your lungs.
"Coronavirus wants to kill you. It wants to take all the life out of your lungs so that you cannot even breathe."
A new centre for assessing suspected coronavirus cases is opening in Londonderry in a bid to prevent hospital wards becoming overwhelmed.
The unit will be based at Altnagelvin Hospital and staffed by GPs.
Patients will be assessed in an out-of-hours urgent care building, and if necessary transferred to specialist wards.
Dr Tom Black, chairman of Northern Ireland's British Medical Association (BMA), said: "This has been shown in Italy and other countries that this is how we save lives."
The triage service is designed for those who are moderately ill and do not need an ambulance to get to hospital.
It is the first of 13 similar centres being established in Northern Ireland, enlisting the expertise of GPs while reducing the pressure on hospital beds and intensive care units.