Professor Sir Patrick Vallance told MPs that more than 50,000 people in the UK may already be infected with COVID-19 - far more than the 1,950 cases confirmed by testing by 12 March.
But fresh measures announced by the government on Monday could help to reduce the peak of the illness by around 50%, he said.
The effects of new social distancing measures, which include telling people to work at home if possible and stop unnecessary travel, would not be seen for two to three weeks, Professor Sir Patrick said.
In evidence to the House of Commons health and social care select committee, the chief scientific officer explained that the UK did not currently have the capacity to test the wider population, but argued he was ‘pressing hard’ to increase capacity.
Healthcare staff testing
He confirmed that the health professionals would be able to access testing when capacity increased, in line with comments from CMO Professor Chris Whitty. Doctors have warned that a failure to provide quick tests could deepen workforce shortages during the spread of the virus.
When asked by health and social care committee chair Jeremy Hunt about reports that social distancing measures could keep the UK death rate below 20,000 people, Professor Sir Vallance, said: ‘That is the hope, that we can get it down to that.
‘And to [compare that with rates for] seasonal flu, the number of deaths is thought to be 8,000 excess deaths. So if we can get this down to numbers 20,000 and below that’s a good outcome in terms of where we would hope to get to with this outbreak.
‘But it’s still horrible, that’s still an enormous number of deaths and it’s an enormous pressure on the health service. And having spent 20 years as an NHS consultant as well as an academic I know exactly what that looks and feels like.’
Increasing testing capacity
Dr Luke Evans, a GP in Oxford and the MP for Bosworth, asked for clarification around when primary care staff would start to be tested for the virus after concerns from GPs.
‘The next group of people I know Public Health England (PHE) and the CMO [want to secure testing for] is key workers, and healthcare workers would be absolutely there. So as the [testing] capacity ramps up, that’s where you would go next,’ Professor Sir Vallance said.
He said the limited number of tests currently available had to be used ‘in the right place’. ‘I think we need a big increase in testing and that’s what I’m pushing for very hard and everyone is working very hard to make that happen,’ he said.
Professor Sir Patrick said experts in Singapore had reported two phases of illness caused by the virus, with patients either recovering after five to seven days or experiencing a second phase that usually kicked in around day five and was characterised by ‘shortness of breath, failure to get better from the first round and then a deterioration’.
He said there was some evidence suggesting that healthcare workers who had experienced this second phase did so because they were 'exposed to very large doses’ of the virus early on.