"Many people were left to die in horrific circumstances", he said during his hours of testimony to lawmakers on Wednesday.
The prime minister, who was described as unfit for office by his former aide, said "some of the commentary I have heard doesn't bear any relation to reality".
He insisted that "at every stage we have been governed by a determination to protect life" but acknowledged the situation in care homes had been "tragic".
"I think it is (wrong)", housing minister Robert Jenrick told BBC radio when asked about the allegation from Cummings that tens of thousands of people had died unnecessarily.
Britain's Health Secretary Matt Hancock arrives for a coronavirus media briefing in Downing Street in London on Thursday. "Quite the opposite of putting a shield around them, we sent people with covid back to the care homes".
Among a laundry list of allegations, Cummings accused Johnson of dismissing the health crisis as "just a scare story", said he was "unfit" to be prime minister, and even suggested at one point Johnson considered being injected with coronavirus live on television to show it wasn't a big threat.
"Cummings rains fire on No 10", read the front page of Britain's Times newspaper, while The Daily Telegraph accused the former adviser of seeking revenge on Johnson and Hancock during the hearing after he branded them "unfit" and dishonest.
Johnson, Cummings said, was unfit for his role, while his greatest criticism was reserved for Hancock who he said had repeatedly lied to such an extent that the country's top civil servant lost confidence in his honesty.
Israeli forces launch air and ground attacks on Gaza as fighting worsens
At the same time, Hamas and Islamic Jihad, a fellow Palestinian militant group, have fired close to 2,000 rockets towards Israel. In addition, strikes killed "a number of terrorists belong to the Hamas naval force", according to a military statement.
He said "it wasn't possible" to test everyone being discharged from hospitals at the start of the outbreak.
"We didn't start with a big testing system in the United Kingdom and then we built that testing system". Mr Cummings said the government's ineptitude and delay had led to many more deaths than necessary.
Shadow health secretary Jonathan Ashworth said the allegations made by Mr Cummings are either true - in which case Mr Hancock "potentially stands in breach of the ministerial code" and the principles of standards in public life - or they are false "and the Prime Minister brought a fantasist and a liar into the heart of Downing Street".
The government says it will begin an independent public inquiry into its handling of the pandemic within the next year.
"Tens of thousands of people died who didn't need to die", he said. Nearly three-quarters of British adults have had one dose of a coronavirus vaccine, and 45% have had both doses.
Speaking about current plans to remove lockdown restrictions on June 21 - a fourth and final step on the government's roadmap out of lockdown - Johnson said that cases of the virus were increasing, including those of a variant first identified in India.
"You have to remember that we didn't have all of the facts at the time that the decisions were being taken".
Asked whether the rise in the Indian variant Johnson said he did not "see anything now in the data" to divert from the June 21 target for the next stage of exiting lockdown but "we may need to wait" for more data.