Sanders was simply better prepared for campaigning to come to a halt, investing for years in a digital video infrastructure that can fire up with no glitches - something the Biden campaign has struggled with.
In his coronavirus responses, Sanders has revisited or expanded on ideas he'd talked about for years, from direct basic income payments to Medicare-for-all.
Biden, who held the vice-presidency during two viral outbreaks, has not used any forum since last week's debate to talk about what he would do.
If Sanders is the neighbour who stockpiled canned food, water, and paper towels, Biden is the neighbour making a last-minute run to Costco.
"Serious question: Where is Joe Biden?" tweeted Carmen Yulín Cruz, the Mayor of San Juan, Puerto Rico, and a national co-chair of the Sanders campaign.
Biden's campaign is adjusting, putting out videos and still sending fundraising emails. The candidate addressed reporters on Saturday with plans for new, daily updates during the crisis, promising that they would "hear more of me than you want to." But he still lags behind Sanders' online presence; the senator held three online discussions about responding to the virus.
Neither candidate has held a campaign event with an audience since March 9, the day before the Michigan primary.
Both candidates are vying to be the oldest man ever elected to the presidency, and both are in the risk zone for exposure to Covid-19. Biden's spare messaging, allies say, does not reflect the hours he spends in touch with congressional Democrats.
"I find myself literally on the phone with my key advisers, medical advisers and economic advisers literally four or five hours a day, going through detailed memorandums on what we should be doing," Biden said in that media call. "My whole focus has basically been how we deal with this crisis."
The reality for Sanders is that he is losing the primary.
Another candidate, in another year, may well have conceded by now.
He is further behind in the delegate count than he was at this point in 2016 and much further than Hillary Clinton had been in her 2008 race against Senator Barack Obama. To erase Biden's lead of around 300 delegates, and the 67 delegates won by candidates who have endorsed Biden, Sanders would need to win nearly two-thirds of the vote in coming primaries, most of them in states where Clinton beat him four years ago.
He would need to do so with a campaign that is unable to hold rallies or in-person events. Sanders has scaled back basic campaign activities while scaling up his messaging efforts. The campaign has cancelled all in-person canvasses, key to its mass organising strategy, replacing them with "virtual phone banks."
It has not solicited money from supporters since March 17, while urging supporters to donate to charities that could help with covid-19 relief, an ask that raised US$2 million.
Biden, outspent so far not just by Sanders but by several candidates who dropped out, has begun to scale up his operation. His campaign was asking supporters for money today, with the candidate himself scheduling a conference call for higher-dollar donors after a similar event last week.
"We wanted to once again reach out and ask for some of your opinions on what kind of communications you want to receive and how you would like to stay involved from home," Biden's email to donors read.
On Sunday, the campaign released an explanatory virus video from Ron Klain, Biden's former chief of staff and the tsar who managed the 2014 Ebola crisis.
Today, it released a side-by-side video of Biden and President Donald Trump taking basically the same question, about what they would say to nervous Americans; Trump blew up at the reporter who asked, while Biden laid out ideas such as a fast expansion of testing facilities. The low-key normalcy, the promise of a campaign that will end and restore the best of pre-Trump America, had helped Biden rout Sanders in the first place.
"I think the contrast in dealing with a crisis like this between an erratic and unreliable Trump and a calm and steady Biden is excellent for Joe," said Representative Brendan Boyle, one of the first members of Congress to endorse Biden.
To Sanders' supporters, the Biden theory of this crisis is not only wrong but dangerous and reason enough to keep the primary going. Some of the focus is on substance, pointing out that Biden would mobilise federal resources and waive patients' bills for this crisis, while Sanders would remake the healthcare system and make those problems irrelevant. Sanders has also torn into Republicans for their responses to the crisis, in greater detail than Biden.
"If you are a couple making US$150,000 a year, you're doing pretty well," Sanders said in a broadcast. "The Republicans would give you a US$2400 checque. But if you are a senior citizen trying to survive on US$13,000 a year from Social Security, you would receive a checque for $600. If you are a college student in 2018 who just got laid off from work last week, you would receive nothing, zero from the Republican proposal. Or you may have virtually nothing in the bank."
Critics on the left see Biden and Democratic leaders as blowing the crisis, as Republicans prepare for trillions of dollars in deficit spending and polls show support for the President's response ticking up. Sanders has proposed several avenues of fast executive action, including one - applying the Defence Production Act to speed up the availability of crucial supplies - that Biden agrees with. But Sanders has been far more visible.
"Personally I think Bernie should turn the whole campaign into a coronavirus response operation," wrote Nathan Robinson, the editor of the left-wing magazine Current Affairs. "It's crucial to keep the primary going, because we have to choose a nominee who can handle the ongoing crisis effectively. If we don't, a lot of people will die needlessly."
The other knock on Biden has less to do with substance and more with mockery.
Prominent Sanders supporters have criticised what they see as his lack of presence on the coronavirus response, promoting hashtags such as #WhereIsJoe, which trended for two days on Twitter; memes of milk cartons or movie posters with the "missing" candidate; and theories of how his 300+ delegate lead could be reversed if Sanders outworked him.
"It's a grave mistake," Shaun King, a civil rights activist and Sanders surrogate, said of Biden's minimal presence. "During a national emergency he just went missing."
The idea that Biden is simply in hiding has been advanced by Randy Bryce, the union organiser who challenged then-House Speaker Paul Ryan, R, in the 2018 Midterms, and Kim Gordon, the former bassist for Sonic Youth. (Bryce has travelled as a surrogate for Sanders, while Gordon has appeared in at least one of his campaign videos.)
Others have speculated that the former vice-president could melt down any moment, using a misleading clip from Biden's March 17 speech to make him look confused. Yesterday, when Biden's account gave a thumb's-up during a live-streamed DJ set, the first comments were speculation that the campaign was finding ways to look active as the candidate himself was hidden.
The Trump campaign was relatively late to react to the mockery, having focused instead on the idea that Biden is being divisive during a national crisis. But it picked up on #WhereIsJoe after the Klain video went out, and worry that Biden is not pressing his advantage - he remains the only candidate who held a position of power during a successfully thwarted pandemic - has permeated with some liberals who support him.
"I want to be in daily or, at least, you know, significant contact with the American people, and communicate what I would be doing," Biden said. "I promise you, that's on the way, hopefully, God willing, by [tomorrow]."
Today the Sanders campaign announced that the senator would hold a live stream on the pandemic, his third since the March 17 primary. For another day, Sanders would put himself in front of cameras, while Biden was still tuning things up.
