Another 616 people who tested positive for COVID-19 have died in hospitals in Britain as of Wednesday afternoon, bringing the total number of coronavirus-related deaths to 18,738, the Department of Health and Social Care said Thursday.

As of Thursday morning, 138,078 people have tested positive for COVID-19, marking a daily increase of 4,583, said the department.

Earlier in the day, the department said some 20,000 households in England are being contacted to take part in the first wave of a major new government study to track coronavirus in the general population.

Led by the department and the Office for National Statistics, the study draws on the world-leading scientific expertise of the University of Oxford, backed by the proven testing capabilities of data science company IQVIA UK and the National Biosample Centre in Milton Keynes.

Participants in the study will form a representative sample of the entire British population by age and geography. The results will help scientists and the government in the ongoing response to the coronavirus outbreak, with initial findings expected to be available in early May, according to the department.

In total, 25,000 people will take part in the pilot phase of the survey, with plans to extend it to up to 300,000 over the next 12 months. Adults from around 1,000 households will also provide blood samples monthly for the next 12 months.

"If you are asked to take part in vital research for this country... If you get a letter, please respond as soon as you can," Health Secretary Matt Hancock said at Thursday's Downing Street press briefing.

Saying that testing capacity has increased to 51,000 a day, he noted that the government was on track to meet its target of carrying out 100,000 tests a day by the end of the month.

"From today, employers of essential workers will be able to go on gov.uk to get a test for any of their staff. From tomorrow, any essential workers who need a test will be able to book an appointment on gov.uk themselves, directly. This all applies for people in essential workers' households too who need a test. It's all part of getting Britain back on her feet," he told reporters.

Meanwhile, a team at the University of Oxford is starting a coronavirus vaccine trial on humans Thursday. The team started work on developing the vaccine to prevent COVID-19 on Jan. 20, 2020.

"This week we will start the process of vaccine evaluation in our first human studies and are currently focusing all efforts on preparing for the start of the trials," the team said Wednesday in a statement.