Thousands of vulnerable patients in Northampton were not told to shield until months after lockdown began last year, figures reveal.
A group of MPs said the Government's lack of planning created a postcode lottery with areas across England reporting unacceptable variations in the number of people shielding.
The Government originally asked those considered to be the most vulnerable to Covid-19 to isolate at home in March last year.
NHS Digital figures show 4,585 patients in Northampton were on the shielding list by April 12 – the earliest version available.
Local doctors were then asked to review the data, causing the patient count to swell to 7,795 by May 15 – an increase of 70%.
Across England, reviews led shielding numbers to increase by 73% – but this varied hugely regionally, from just 15% in Carlisle to 352% in the London borough of Hounslow.
The Commons Public Accounts Committee said the "unacceptable" level of variation was evidence of a postcode lottery, as the list grew from 1.3 million to 2.2 million nationally.
Committee chairwoman Meg Hillier said this was caused by "poor data" and a lack of "joined-up policy systems".
She said: “The shielding response in the Covid pandemic has particularly exposed the high human cost of the lack of planning for shielding in pandemic-planning scenarios.
“People were instructed to isolate, to protect themselves and others – but the cost of this protection was reduced access to living essentials like food, and an untold toll on mental health and wellbeing.”
MPs also found that a contact centre established to trace those who did not respond to an initial shielding letter was unable to reach 800,000 people – and it is still not known whether they had been contacted by local authorities.
Clinically extremely vulnerable patients in England have not needed to isolate since April 1, by which point over 90% had received at least one vaccine dose.
The national shielding list increased significantly again in February, when scientists developed a new tool to assess whether someone is at risk of severe disease or death.
The latest update shows the number on the shielding list in Northampton has risen to 16,010 as of April 14 – 11,425 more than a year previously.
The British Medical Association said inadequate planning and unclear Government communication made shielding a distressing experience, and jeopardised the public's health and wellbeing.
BMA council chair Dr Chaand Nagpaul added: “The shielding lists could have been created more quickly and effectively rather than shifting criteria and suddenly increasing the number of people on the list.
"This led to a huge rise in workload having to contact large numbers of people within a short time frame without comprehensive guidance."
The Government said it delivered an urgent national scheme to contact people and provide them with vital food and medicine so they could shield – but many chose not to take up the offer.
A spokesperson added: “These report findings are disappointing and misjudged.
“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.”