From a crowded pool party at Lake of the Ozarks in Missouri to a street party in Daytona Beach, Florida, Memorial Day festivities brought hundreds close together in several U.S. cities Monday ⁠— largely without masks or other such precautions.

But while behaviors related to the pandemic have changed in recent days, the director of the Vaccine and Immunotherapy Center at Massachusetts General Hospital said the coronavirus itself is still very much the same.

“The virus is what it was previously: highly infectious and transmissible from one person to others over short distance,” Dr. Mark Poznansky told Jim Braude on WGBH News’ Greater Boston Tuesday.

“There could be some perfectly healthy-appearing people in that pool, who are, unbeknownst to them, spreading the virus liberally to other people who are close to them,” he added.

Poznansky, who is also a practicing physician at Mass General and an associate professor at Harvard Medical School, warned that outbreaks across the country will only get worse if people disregard public health warnings in the name of personal freedom.

“We are extremely fortunate to live in a free country where people can express their independent views wherever they are, whether it’s on a beach or in the public town square. But the truth is, about an infectious disease, it knows no boundaries,” he explained. “A person sitting on a beach, talking about their own concerns about their own health and their own fatalistic approach, doesn’t mean that they may not be impacting ... someone else sitting along that beach.”

The need for such social distancing measures may be around longer than some public health officials have suggested. Poznansky said the vaccine Mass General’s Vaccine and Immunotherapy Center is working to develop ⁠— one of more than 100 different efforts in progress ⁠— won’t be ready for widespread use at the end of the 18-month timeline several experts have put forth as a possibility.

“We’re entering animal testing next month and within 6 to 9 months, we’ll be entering first in human testing if all goes smoothly,” he said of the vaccine candidate called HaloVax, which is a joint effort between Mass General and the pharmaceutical company Voltron Therapeutics. “[It will take us] well into 2021 for sure, to even enter into the second phase of human testing.”

Still, he said potential coronavirus vaccines have been moving through the phases of development at an “unprecedented speed.”

“There are companies now which are actually manufacturing large quantities of vaccine even as they’re beginning trialing that vaccine,” Poznansky said. “And that’s one indicator of an accelerated timeline which I don’t think we’ve actually ever seen before.”