Here’s what we know about the Delta variant in Mass.

"If you’re worried about the variants, get vaccinated."

Alice Kobres received the last COVID-19 vaccination at Gillette Stadium. (Christiana Botic for The Boston Globe)

As the Delta variant has spread, so too has anxiety and uncertainty about possible risks and what public health advice to follow. Area infectious disease experts say it comes down to two things: heeding local guidance and, most importantly, getting vaccinated.


The primary concern about the Delta variant is that it’s more contagious than other strains, but what exactly makes the virus more transmissible is still being researched. Evidence suggests that the Delta variant, like other COVID strains, poses the most significant risk to unvaccinated populations.

Public health officials told the New York Times Delta could be 50% more contagious than the Alpha variant, though estimates vary, and is believed to be the most transmissible variant yet. This could be because it can partly evade antibodies from infection or vaccination — though studies show vaccination still protects against serious illness — or it’s better at binding to our cell receptors. Delta may also cause more serious illness in unvaccinated individuals.

How bad is the Delta variant in Massachusetts?

Massachusetts doesn’t sequence every positive COVID-19 test, so it’s difficult to definitively know how widespread the variant is. However, since community transmission remains low statewide, it would be a percentage of an already small number of cases (though transmission can vary between cities and even neighborhoods).


The CDC reported that the Delta variant represented 30.4% of cases nationwide between June 6 and 19. In Massachusetts, Maine, New Hampshire, Connecticut, Vermont, and Rhode Island, the Delta variant represented 12.2% of cases. However, more recent reports indicate the nationwide percentage is much higher: a CDC spokesperson told Politico that surveillance data shows the Delta variant represented 51.7% of cases June 20 to July 3.

This graphic shows the variant breakdown over time. It’s clear the Delta variant is quickly spreading. – Center’s for Disease Control and Prevention (

A state Department of Public Health spokesperson told that 3.32% of COVID-19 cases in Massachusetts have been sequenced — a process that determines the strain of COVID-19 — since the start of the pandemic. Since the middle of January, the state has sequenced 23,561 cases, representing 10% of cases during that period. However, the DPH did not indicate how many of those cases were the Delta variant.

Low vaccination rates could be another indicator of high levels of Delta cases, as COVID-19 vaccines have been shown to be effective against the Delta variant. Massachusetts has high vaccination rates; as of July 6, 78% of the eligible population in Massachusetts have at least one dose, and 70% are fully vaccinated.

How should the Delta variant impact policy?

In the wake of increased transmission, some officials have made the call to reintroduce mask wearing indoors. On June 14, the World Health Organization published updated considerations on public health measures around COVID-19. It recommended that everyone — including vaccinated individuals — continue wearing masks when there are certain levels of community transmission.


“Knowing the level of transmission is key to assessing the overall COVID-19 situation in a given area and guiding decisions on response activities and tailoring epidemic control measures,” the report reads.

Shortly after, Los Angeles recommended that “everyone, regardless of vaccination status, wear masks indoors in public places as a precautionary measure,” the New York Times reported. The recommendation came amidst an increase in infections and Delta variant cases and persistently high numbers of unvaccinated residents, notably children, Black and Latino residents, and essential workers.

The CDC hasn’t made such a call nationwide, but Dr. Sabrina Assoumou, an infectious diseases physician at Boston Medical Center and assistant professor of medicine at Boston University School of Medicine, believes both decisions are appropriate. To Assoumou, what’s happening in L.A. isn’t necessarily relevant to Boston: the best decisions we can make are based on local transmission data and public health guidance.

“Measures should be localized,” she told “Look at the rest of the world; there’s ongoing transmission and not enough vaccinations, so in those cases, yes, masking indoors makes sense. Whereas if you take a state like Massachusetts… we’ve done a really good job, and that’s reflected in the number of new cases and test positivity which is less than one percent. We should continue to encourage people to get vaccinated, but our recommendations should fit what we’re seeing, and I think masking indoors may not be necessary.”

Crowds gather on L Street Beach, Saturday, June 5, 2021, in the South Boston neighborhood of Boston. – (AP Photo/Michael Dwyer)

What would indicate a need to return to those restrictive measures? Dr. Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women’s Hospital, told there would need to be evidence of vaccine failure.


“If we’re seeing evidence that a variant is causing a significant number of infections among vaccinated individuals, particularly if those infections are leading to hospitalization, that would be a reason to reinstate widespread masking for indoor activities,” he said. “We’d have to have a reasonably high threshold for reimposing those restrictions given the economic impact. We can’t just do it because we’re seeing infections predominantly in unvaccinated populations.”

The rate of breakthrough cases is incredibly low, the DPH spokesperson said, and cases where the person was hospitalized or died are even lower. The DPH reported that as of July 3, 0.1% of vaccinated individuals in Massachusetts tested positive for COVID-19; 4,175 out of 4.1 million. Of those, 92.34% did not result in hospitalization or death; 0.55% died only, 5.82% were hospitalized only, and 1.29% were hospitalized and died.

If residents want to know what they can be doing to stay safe, Assoumou had a simple answer: get vaccinated.

“We are just so fortunate to have these vaccines, and they work really, really well,” she said. “The message should be: ‘if you’re worried about the variants, get vaccinated.'” 

“I know that a lot of people are getting scared and nervous, so if you’re scared and nervous, get vaccinated because that’s your sure way to protect yourself, protect your family, and protect your community. We all have a role to play.”


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