He tweeted this was a “very important issue re learning from the disaster” in response to a thread highlighting how Vietnam had successfully protected its borders.
His tweet followed a lengthy blog post yesterday where he openly attacked Mr Johnson.
He accused the PM of trying to block an inquiry that implicated Henry Newman, a friend of his fiancée Carrie Symonds, after details of the autumn lockdown were leaked to the press.
Mr Cummings also claims to have warned Mr Johnson that using donors to pay for renovations of the Downing Street flat was “possibly illegal”.
He wrote: “It is sad to see the PM and his office fall so far below the standards of competence and integrity the country deserves.”
The blog and follow-up tweet came just a month before Mr Cummings is due to give evidence to MPs on May 26 about the Government’s handling of Covid.
Mr Cummings was a key figure helping steer Britain’s Covid response, having helped deliver Brexit for the Johnson administration before he was forced our in December following a period of infighting.
Lax border restrictions have been blamed for Britain’s crippling second wave which saw the country plunged into lockdown for months.
Hundreds of thousands of travellers continued to come to Britain without being tested or undergoing self-isolation.
Scientific Advisory Group for Emergencies (SAGE) papers released last summer reveal that at the height of the first wave ministers were told that shutting the borders had “little scientific justification”.
But Mr Cummings today retweeted a post from a “superforecaster” called Michael Story who highlighted research that described such advice as “evidence-free”.
Mr Story said: “The border belief described here was incredibly powerful and wormed its way around institutions the world over.
“The pandemic preparedness index ranked you *less* prepared if you had previously closed borders in the event of disease outbreaks.”
The Twitter thread also upheld the approach taken by Vietnam’s communist government of sealing itself off, pointing to its comparatively low 35 reported deaths.
It was not until June that Home Secretary Priti Patel announced that all international arrivals would have to spend 14 days in quarantine.
Now foreign travel is illegal except in exceptional circumstances and all arrivals must undergo quarantine.
Those coming from red list countries are forced to self-isolate in government-approved hotels.
Last month Mr Cumming’s described the Department for Health and its pandemic preparedness as a “smoking ruin”.