Amazon workers can take unlimited sick days this month - but only those who test positive for the Covid-19 virus will be entitled to any sick pay.
Many US warehouse workers are entitled to only 10 days' leave a year - for all holidays, sickness and emergencies.
And some had asked Amazon to change its policy to stop coronavirus spreading.
Some workers also say they are being told increase their hours, known as mandatory extra time, because of increased demand and staff shortages.
In a blog, Amazon said: "The health and safety of our employees and contractors around the world continues to be our top priority as we face the challenges associated with Covid-19.
"Effective immediately, all Amazon employees diagnosed with Covid-19 or placed into quarantine will receive up to two weeks of pay.
"This additional pay while away from work is to ensure employees have the time they need to return to good health without the worry of lost pay.
"This is in addition to unlimited unpaid time off for all hourly employees through the end of March."
Amazon has also established a $25m (£19.6m) fund to support independent delivery service partners and their drivers, Amazon Flex participants and seasonal employees "under financial distress".
BBC News has had access to an online forum used by about 25,000 Amazon warehouse workers.
And some of those unaffected by coronavirus are clearly relieved they will no longer have to struggle into work when either they or their family are sick.
"My two kids have been sick and last night I felt like death. Now, I can take time off and help the three of us to get better," wrote one.
Another said: "It's allowing me to take a break and get my blood pressure down. My vision started to get blurry... my blood pressure was 176/100[mmHg]."
Those who test positive for the virus will now be guaranteed two weeks' pay.
And one Amazon warehouse worker told colleagues on the forum, the company's previous policy of not offering sick pay had been dangerous.
"Many co-workers live pay cheque to pay cheque and are still going to come into work even if they're sick, which means that coronavirus will spread like wildfire through Amazon, just like all the other sicknesses do," they wrote.
One in four US workers - more than 32 million - is not entitled to paid sick days and, according to the Bureau of Labor Statistics that rises to seven in 10 among low-paid workers.
And even before the coronavirus crisis, Amazon workers had begun to demand better conditions, with more paid time off (PTO) being chief among them.
In some facilities, workers have begun wearing badges saying: "Amazonians united for PTO".
"Amazon thinks we're just some stupid workers. But we know our rights, we're organising and we're winning," one worker, who asked to remain anonymous, told BBC News.