Bosses said workers at sorting offices had been instructed to look out for letters containing the NHS logo and prioritise them for delivery.
It comes as parts of the West Midlands have suffered reduced postal deliveries in recent weeks – mainly due to Royal Mail suffering a high number Covid-related staff absences – prompting concerns over delays to the delivery of vaccination letters.
David Gold, director of public policy at the Royal Mail Group, was contacted by Lichfield MP Michael Fabricant over the matter.
Mr Gold said: “Covid is having a big impact in Lichfield and elsewhere, less because of infections per se, but more due to the need for people to isolate on advice from Track & Trace.
“We are working closely with public health authorities and other letter carriers to play our part in the delivery of vaccination letters.
"We recognise the vital importance of delivering vaccination letters as quickly as possible. We would like to reassure you that these vaccination letters are being prioritised.
"Our people have been instructed to look out for the vaccination letters which are identified by an NHS logo so that they can be prioritised during the sorting process."
Mr Gold said Royal Mail was getting daily updates on the number of vaccination letters going out, enabling them to plan for delivery.
"This ensures we can focus our resources to deliver these letters as quickly as possible," he added. "In areas where Royal Mail offices are experiencing particularly high levels of Covid-related absences, we will draft in extra resource to help prioritise vaccination letters still further."
Mr Fabricant said constituents had raised concerns after postal deliveries in some areas had "stopped altogether".
But he pointed out that if people did not respond to letters they would be contacted electronically – by text, email, or a phone call – from their GP practice inviting them for a vaccination "when the time is right".
He added: "People should not worry about losing out."