During a Downing Street briefing on Monday, he insisted the figures were not a prediction, but warned: "At the moment we think the epidemic is doubling roughly every seven days".
Antibodies can protect against the virus, but their efficacy also fades over time and people have been reinfected in some cases, according to Vallance.
It comes as influential Tory MP Sir Graham Brady signalled that ministers could face backbench resistance if they try to introduce new lockdown measures without proper scrutiny in Parliament.
Health Secretary Matt Hancock refused to guarantee that pubs in England would be allowed to open this weekend as ministers considered options to limit the spread of the virus.
He said as the disease moves up the age groups, the mortality rate would move to significant rates, although drugs such as dexamethsone would reduce the mortality rates a little.
"But if we go too far the other way, then we can cause damage to the economy which can feed through to unemployment, to poverty, to deprivation - all of which have long-term health effects, so we need always to keep these two sides in mind".
He said he feared cases could go "shooting through the roof" with more hospitalisations and more deaths.
"The situation is clearly worsening", a spokesman for the mayor said.
"I'm a Member of Parliament as well as a minister so I don't disagree about Parliament's role in all of this. It is better for both health and business to move too early than too late".
People in England who refuse to self-isolate to stop the spread of coronavirus could face fines of up to £10,000 ($13,000, 11,000 euros) under tough new regulations announced Saturday.
"We should see this as a six-month problem that we have to deal with collectively", he said.
The latest daily United Kingdom figures show that there have been 4,368 new Covid cases, and the latest 24-hour death figure is 11, bringing the total official number of deaths from the virus to 41,788.
Nearly 42,000 people who have tested positive for Covid-19 have died in Britain, the worst death toll from the pandemic in Europe. Described as a "circuit break", the aim would be to impose tighter restrictions across England to curb the chain of transmission before it accelerates further.
After a slow rise in COVID-19 infections over the summer, the number of new cases is now doubling every seven days, and if nothing is done to slow the spread of the disease this could lead to as many as 49,000 cases a day by mid-October, Chief Scientific Officer Patrick Vallance told the public during a televised briefing.
"But we of course have to be prepared to take action if that's what's necessary".
Sir Keir Starmer said Labour would support any new measures but warned that a second national lockdown was becoming more likely because the Test and Trace programme was in a state of "near collapse".
People on benefits will be eligible for a one-off support payment of £500 if they face a loss of earnings as a result of being required to quarantine.