MPs have called on Dido Harding, the head of NHS test and trace, to prove that the system substantially reduces the spread of coronavirus as efforts continue to contain more transmissible variants she claimed no one had seen coming. Harding told the Commons science and technology committee on Wednesday that NHS test and trace was on track to reduce R, the reproduction number of the epidemic, by 0.6 to 0.8 at the end of March, an improvement over the 0.3 to 0.6 reduction she said was achieved in October. But in a sometimes testy exchange, Greg Clark, chair of the committee, pressed Harding on why she had failed to publish evidence for the claim, noting she had told the public accounts committee last month that the assessment would be published “soon”. Harding defended the delay, saying the information needed to be “properly quality assured”, but expected it to appear in the next couple of weeks. Pointing out that this would mean more than a monthlong wait, Clark said: “For an organisation for which the watchword should be speed, this doesn’t seem to be a feature of your disclosure of the evidence.” Harding responded: “I completely disagree.” The reproduction number, R, reflects how many people an infected person spreads the disease to and must be below one for the epidemic to wane. Scientists estimate that the R number across the UK is between 0.7 to 1.1 under the current lockdown. In the autumn, the Scientific Advisory Group for Emergencies (Sage) concluded that NHS test and trace was having only a marginal impact on the epidemic. But Clark said an effective test-and-trace system and reliable information on its performance was crucial for decisions on how lockdown can be lifted. “If it’s a significant reduction in R that is being made that’s great, we’ll all welcome that and take confidence in it, but we do need to know what it is,” he said. Harding was further pushed to defend the business case she put forward in September for an extra £15bn for NHS test and trace. At the time she argued that the “main driver” for the justification was the avoidance of a second national lockdown. The second lockdown came in November followed by a third in January. Harding told MPs that while everyone wanted a “straightforward route out of the Covid crisis”, test and trace was “not a single silver bullet” and that between the business plan being published and the UK going into the latest lockdown, the virus had mutated. “We’ve seen the new variants emerge, which was something that none of us were able to predict,” she said. The claim that more transmissible variants were not foreseen was later criticised by Jonathan Ashworth, shadow health secretary, who pointed out that scientists have long warned of new variants as the pandemic unfolds. Harding clarified that she was referring to when new variants would emerge. “I don’t think any scientist in the world would sign up to a date stamp of when particular mutations are likely to occur, and it’s why it’s so important that we have surge capacity in our test-and-trace system,” she told the committee. About 20,000 people per day who are contacted by test and trace are not fully isolating, Harding said. The estimate, drawn from last week’s figure of 700,000 cases and contacts, and surveys showing that 20% to 40% of people cannot, or do not, fully comply with isolation rules, was enough to restart the pandemic, according to Jeremy Hunt, the former health secretary. Harding said new variants, such as those first spotted in the UK, South Africa and Brazil and the rollout of vaccines presented a fresh challenge for test and trace this year. More transmissible variants mean a faster turnaround time “becomes more an imperative”, she said, while widespread vaccinations would drive up asymptomatic infections and the need for mass testing to contain outbreaks. She stressed that as the peak of the pandemic passed, routine testing and surveillance for new variants would need to become part of every day life, while Britons had to lose the “stiff upper lip” that led people to go to work with colds and flu.