Weeks of low death numbers mean the British public has lost sight of how important Covid-19 restrictions are in controlling the virus, epidemiologists have warned.

But deceptively low coronavirus death numbers could be lagging weeks behind the sharp increase in cases in the UK, they fear. Until Tuesday, the daily death count – which once regularly topped 1,000 – had not edged above 20 for weeks.

The government on Tuesday night announced the most significant toughening of lockdown in months, with gatherings of more than six people banned indoors and outdoors.

It comes amid an uptick in coronavirus infections, with the UK reporting 8,400 new cases of the virus in three days, and recording 32 deaths on Tuesday – the highest in six weeks, but so far an isolated jump.

“People have got a little bit tired of the restrictions,” said Professor Hynek Pikhart, an expert in epidemiology and medical statistics at UCL. “I don’t think they are following them as strictly as they were in April and May. 

“I think when you see thousands of deaths, you behave differently to when you see three or four a day.”

Photos of people on “packed in” streets over the summer – the government encouraged people to begin socialising and returning to pubs and restaurants in July – left him “horrified”.

It is also likely that people travelling to countries with higher levels of coronavirus for holidays brought cases back to the UK, he said. 

With Covid-19 cases on the rise once more, it is as vital as ever for people to follow social distancing rules and wear face masks in public, Pikhart said. 

On Sunday, 2,988 new cases of coronavirus were reported in the UK – the biggest daily jump in infections since May 22. On Monday, another 2,948 cases were recorded. 

An increase in the UK’s coronavirus testing capacity means it’s not possible to draw exact comparisons with figures from three months ago – but health secretary Matt Hancock has branded the rise in cases “concerning” nonetheless. 

The number of people dying from Covid-19 in the UK remains low compared to the peak of the pandemic – as does the number of people hospitalised by the virus – but this is likely to change if infections continue to rise, Pikhart warned. 

In August, a leaked report from Sage – a group of scientists which advises the government – warned that 85,000 people could die from Covid-19 in the UK this winter in a “reasonable worst case scenario”. 

The coronavirus pandemic is a “marathon”, not a sprint, Pikhart added. “This winter will be critical.” 

It is a warning echoed by Jasmina Panovska-Griffiths, a senior mathematical modeller from the UCL Institute of Epidemiology and Health Care. 

“There is a danger that the number of hospitalisations and deaths may be lagging behind [the recent spike in cases] and we just don’t know at the moment,” she said. 

“This is the concerning factor – how is this going to develop over the next couple of days, weeks and months?” 

It’s important to remember that the virus hasn’t gone away, Panovska-Griffiths warned.

“We have suppressed it with social distancing and with other interventions like face coverings, but it hasn’t gone away. 

“Until we know there is an effective vaccine or antiviral drug, we can’t be certain.”