"And there's still a large group of people who are anti-vaxxers or just don't listen to the facts. It's a real shame.
"I've just lost a few people in my weekly routine who have refused, or did not disclose [whether they'd been vaccinated], and it was unfortunate.
"I feel it's your moral and professional obligation to inform, since we're not all podded up and being tested every single day.
"It's tricky, because everyone is entitled to their own opinion, but a lot of opinions don't feel based in anything except fear or propaganda."
Aniston got some pushback from her followers on social media, some of whom asked why she was so worried about the vaccination status of her friends, given she had already got the vaccine herself.
She highlighted one such comment on her Instagram story, and explained herself further.
"But if she's vaccinated she's protected correct? Why be worried about unvaxxed around her?" the commenter asked, adding a shrug emoji.
"Because if you have the variant, you are still able to give it to me," Aniston said.
"I may get slightly sick but I will not be admitted to a hospital or die. But I can give it to someone else who does not have the vaccine and whose health is compromised (or has a previous existing condition).
"Therefore I would put their lives at risk. THAT is why I worry. We have to care about more than just ourselves here."
She also shared a photo of some embroidery which said: "What doesn't kill you mutates and tries again."
Health experts worry that if the virus is allowed to continue spreading through a large number of unvaccinated Americans it could mutate again, eventually spawning a variant more resistant to the vaccines.
This isn't the first time Aniston has waded into the Covid culture war. She advocated for social distancing and other restrictions in the early months of the pandemic, and in June of 2020 she told her fans to wear face masks.
"I understand masks are inconvenient and uncomfortable. But don't you feel that it's worse that businesses are shutting down, jobs are being lose, healthcare workers are hitting absolute exhaustion?" she said at the time.
"And so many lives have been taken by this virus because we aren't doing enough.
"I really do believe in the basic goodness of people, so I know we can all do this, but still, there are many people in our country refusing to take the necessary steps to flatten the curve and keep each other safe.
"People seem worried about their 'rights being taken away' by being asked to wear a mask. This simple and effective recommendation is being politicised at the expense of people's lives. And it really shouldn't be a debate.
"If you care about human life, please just wear a damn mask, and encourage those around you to do the same."
Half of the American population is fully vaccinated, and about two-thirds of people have received at least one dose.
The country's Covid death toll stands at 632,000.
