Singh had taken both the doses of COVID-19 vaccine.
'We must resist the temptation to look at the absolute numbers being vaccinated, and focus instead on the percentage of the population vaccinated,' he said in his letter.
The interrogation of K T Jaleel is the first time that an incumbent minister has been questioned by the National Investigation Agency (NIA) since its formation in 2009.
Noting that disinformation is no substitute for diplomacy or decisive leadership, Singh called upon the prime minister to ensure justice for soldiers who died defending India's territorial integrity.
Singh is currently a Member of Rajya Sabha from Rajasthan. He was the prime minister between 2004 and 2014.
Singh, 87, was admitted to the hospital on Sunday evening after he complained of uneasiness.
The finance ministry on Thursday froze inflation-linked allowance for 50 lakh central government employees and 61 lakh pensioners till June 2021 due to the COVID-19 crisis.
Addressing a meeting of the Congress Working Committee, Gandhi said it should worry every Indian and her party will have to work hard to repair this damage.
Former Prime Minister Manmohan Singh has warned that communal tensions, economic mismanagement and COVID-19 virus scare are threatening India's progress.
Leaders paid tributes to late Prime Minister Indira Gandhi on her 35th death anniversary and remembered her contribution to national security, economy and foreign policy.