Even as the highly contagious delta variant continues to dominate cases in Massachusetts and amid growing reports of breakthrough infections, Boston-based doctors say their faith in the efficacy of COVID vaccines hasn't wavered.
Getting vaccinated is still the best way to protect people against severe infection, hospitalization and death caused by the virus, local experts asserted, citing the various causes of breakthrough cases and rare deaths as well as evidence of the vaccines' ability to withstand variants -- including delta -- that develop.
"Bottom line, I still have confidence in the vaccines," Boston Medical Center's Dr. Davidson Hamer wrote in an email.
The delta variant accounted for about 75% of coronavirus cases reported in New England as of July 17, the U.S. Centers for Disease Control and Prevention variant tracker shows. But that number has likely risen to 90% or more in Massachusetts by now, Hamer said this week.
Get Boston local news, weather forecasts, lifestyle and entertainment stories to your inbox. Sign up for NBC Boston’s newsletters.
Meanwhile, fully vaccinated people are still getting infected with COVID, though to a much lesser extent than unvaccinated people. In the last week, 2,232 new breakthrough cases were reported Tuesday, bringing the total number of infections in people who have been vaccinated to 9,969.
Dr. Daniel Kuritzkes, chief of Brigham and Women’s Hospital's infectious diseases division, explained a multitude of reasons why breakthrough infections are being reported.
"The reasons why we’re seeing cases in vaccinated people are complicated, and probably due to more than one factor," Kuritzkes said via email. "Even if the vaccine is 95% effective, in a setting where a majority of people are vaccinated, in the setting of a surge, it is inevitable that a significant number of diagnosed cases will occur among the vaccinated."
The highly contagious nature of the delta variant could also be contributing to a greater risk of breakthrough cases, Kuritzkes added, as well as some waning of immunity in people who were among the first to be vaccinated.
More than 100 fully vaccinated people had died from the disease in Massachusetts by the end of July, the Department of Public Health reported. The deaths represent a tiny fraction -- about 0.002% -- of all fully vaccinated individuals in the state, according to state health officials.
"Deaths in people who have been vaccinated are very rare, but in this period where delta is dominant, there is more transmission happening and a higher risk for vaccine breakthroughs if people are not careful," Hamer said.
The doctors noted that there have been a series of reports showing different numbers, pointing to studies out of England and Israel that suggest nearly equal protection against delta as against alpha variants, if not slightly less. Overall, though, the experts said getting vaccinated remains the best option to protect people against severe infection, hospitalization and death caused by the virus.
"Because a greater proportion of recent infections are in unvaccinated people relative to those who have been vaccinated, our best strategy remains attaining a high level of vaccine coverage," Hamer said.
A number of recent publications and studies have shown that all three vaccines approved for emergency use by the Federal Drug Administration still work against the delta variant, Hamer noted.
"They appear to have slightly lower effectiveness in preventing symptomatic infection but they still work and they remain highly effective in preventing hospitalizations and severe disease," Hamer said.
Kuritzkes pointed to a study from Public Health England, which showed "just a very slight decline," in protection by the Pfizer vaccine against the delta variant as compared to the alpha variant.
Carried out from April to May, the study out of England found that, after two doses, the Pfizer vaccine was 88% effective against symptomatic disease caused by the delta variant, CNBC reported.
"That study included a very large number of people, which is a strength; the main weakness is that the time from vaccination was shorter than in the smaller study from Israel that showed lower protection," Kuritzkes said, adding that the vaccine still provides, "strong protection against the worst outcomes."
Preliminary data in Israel — which had one of the fastest vaccination programs in the world — published in late July found that the Pfizer COVID vaccine was just 40.5% effective at preventing symptomatic disease.
"It appears that the vaccines lose only a few percentage points in terms of effectiveness against infection, symptomatic infection, hospitalization and death, with retained effectiveness against hospitalization of over 95%," said Dr. Shira Doron of Tufts Medical Center. "Given these data, the reason we are hearing about more breakthrough cases and seeing more breakthrough hospitalizations is strictly a numbers phenomenon. When there are more cases around, populations that are proportionally spared will get sick in larger numbers even if the proportions stay the same. This is also why we are seeing more sick kids."