Up to 800,000 shielding people could have "slipped through the net" of Covid support, a damning report finds today.
The government still has "no knowledge" how many extremely vulnerable people missed out on help such as food parcels in the first wave of the pandemic, the Commons Public Accounts Committee warned.
Around 2million shielders were contacted through letters, e-mails and then calls from a £18.4m contact centre to offer support.
But the centre run by the Ministry of Housing, Communities and Local Government was unable to reach 800,000 of them, "despite apparently making hundreds of thousands of calls every day," today's report finds.
People's details were later passed from central government to councils, which followed up again whether they needed help.
But "crucially, MHCLG has no knowledge of whether [councils] then managed to reach any or all of these people," MPs said.
That means while many of the 800,000 'missing' shielders will have been helped, the government could not say how many were.
The 800,000 included roughly 375,000 people whose phone numbers were missing or incorrect in NHS records.
Today's report also slams a "postcode lottery" that emerged after people were first told to shield in March 2020.
By mid-April last year, NHS Digital had drawn up a list of 1.3 million people in England who should be shielding.
But this ballooned to 2.2million by May 7 as GPs added individual patients to the shielding list.
Because the decision was left to individual GPs, people in some areas were far more likely to be added than others.
The size of each council area's shielding list grew by anywhere from 15% to 352% depending on where they were in the country.
The committee said this was an "unacceptable level of variation" between different areas.
PAC chair Meg Hillier said the shielding programme exposed "the high human cost of the lack of planning".
She added: "There are questions still to be answered about the balance between central decision making and local knowledge. "Plans were eventually, sensibly devolved to local authorities.
"There needs to be a clear plan ahead for those with serious health conditions so they can access the support they need when they have no other support network.”
A Government spokesperson said: “These report findings are disappointing and misjudged.
"During this globally unprecedented emergency, we worked across multiple government departments to build and deliver an urgent national scheme in record time, identifying 1.8 million clinically extremely vulnerable people and providing them with vital food and medicine to help them shield effectively.
“We made significant efforts to contact people by letter, text and telephone and worked closely with councils to ensure we reached them. Many people chose not to take up the offer of government support as they felt they didn’t need it.
“The initial shielding guidance was agreed by the four UK Chief Medical Officers on the basis of the latest available evidence. Since then we have learned more about the virus and adapted our approach, which has enabled us to protect those most vulnerable by providing them with shielding guidance and prioritising them for vaccination.”