88-year-old Singh was admitted to the All India Institute of Medical Sciences (AIIMS) here after he was infected with contagious disease.
The veteran Congress leader suffers from heart-related ailments, sources said.
Yesterday, Singh had written to Prime Minister Narendra Modi on the current coronavirus situation in the country.
In his letter, Singh offered five suggestions to ramp up vaccination programme, urging the government to be transparent in publicising data on stocks and their distribution patterns.
He said the Centre should grant greater flexibility to States to decide who to designate and vaccinate as frontline workers even if they are below 45 years, and support vaccine manufacturers with funds and concessions.