Two hundred of the more than 375,000 D.C. residents who are fully vaccinated against the coronavirus have tested positive for the coronavirus and four have died, according to a dashboard District officials unveiled Thursday.

But officials said the small number of “breakthrough” cases shows that the vaccinations are effective in preventing illness in the vast majority of cases.

The four fully vaccinated people who died of covid-19 had significant medical histories, and three of them were over the age of 65, Ankoor Shah, head of the city’s vaccine program, said Thursday. Of the 200 breakthrough cases, 57 percent had symptoms, 28.5 percent were asymptomatic. (For 14.5 percent, symptom status was not known.) Thirteen people were hospitalized with covid-19, or 6.5 percent of the breakthrough cases.

“This does not mean that the vaccines did not work for these individuals,” Shah said, adding, “The really great thing about our vaccines is how effective they are in preventing hospitalizations, severe disease and death.” Without vaccinations, the asymptomatic people may have had symptoms and the symptomatic people may have had more-severe ones, he said.

Whether someone contracts the virus after being vaccinated depends in part on the sorts of activities they engage in and their exposure to the virus, such as through travel or work.

“It is absolutely expected that there are breakthrough cases with any vaccine,” Shah said. “No vaccine is 100 percent effective, and we know this coming in.”

Just 0.05 percent of the people who were fully vaccinated with the Pfizer-BioNTech vaccine tested positive, compared with 0.04 percent of those who received the Moderna vaccine and 0.12 percent of those who got the Johnson & Johnson shot.

Through contact tracing, the city learned that the highest number of breakthrough cases were in the 30-to-49 age range, representing 36.5 percent of the cases. No breakthrough cases have been reported for people under 18. Fifty-eight percent were women. And 46.5 percent of the breakthrough cases were Black residents, compared with 35.5 percent who were White and 7.5 percent who were Hispanic. 

The dashboard will be updated every two weeks, and the city plans to eventually include geographic data to better understand the breakthrough cases. Officials said they will also include data on how many of positive cases are from people who are unvaccinated.

The city also announced Thursday that laboratories would be required to conduct additional sequencing of positive coronavirus tests to look for variants.

While 83 percent of all samples in the United States show the highly contagious delta variant, just 1 percent of samples in D.C. show the variant, said Patrick Ashley, the senior deputy director of emergency preparedness and response at the D.C. Department of Health.

It is unclear how comprehensive the city’s sequencing is, leading the city to institute the rule requiring labs to look for the variants.

“Prior to this rule being in place, laboratories had a requirement to report positive cases to us but they did not have a requirement to sequence those cases,” Ashley said. “What this new rule does is put a requirement in place that either the clinical laboratory sequence a proportion of their samples or they transmit those and physically ship those samples to our D.C. public health lab so that we can do the sequencing on those samples.”

Officials said 1,613 samples from D.C. residents who have tested positive, mostly over the past few months, have been sequenced, with 16 of those samples showing the delta variant. Officials were unable to say how many, or whether a majority, of laboratories were already doing that sequencing before Thursday’s new rule. Ashley noted that not every laboratory had the ability to sequence and not every sample could be sequenced, such as those coming from rapid tests.

The seven-day average of new daily cases in the Washington region continues to increase, with the delta variant already the dominant strain in Maryland and Virginia. The seven-day average for the three jurisdictions stood at 740 on Thursday, up from 454 one week ago — although still much lower than the 8,698 it was in early January, at the last peak of the pandemic.

The seven-day average of new daily deaths in the region was seven on Wednesday, compared with six one week ago.

Maryland reported a dramatic spike Thursday with 303 new cases, nearly 100 more cases than the state recorded a day earlier. Thursday’s number represents a 206 percent increase over the 99 new daily cases that were reported two weeks ago.

There have been 153 cases of the delta variant in Maryland since the state began tracking that strain of the virus, state health officials said this week.

Health officials said nearly all new cases, deaths and hospitalizations in the state are among those who are unvaccinated.

Younger people make up the majority of the new cases in Maryland, with 192 of the new cases among people who are under the age of 40. One in 5 of the new cases are among Marylanders between the ages of 20 and 29.

The state’s seven-day positivity rate is also on an upswing, at 1.73 percent. Prince George’s and Montgomery counties, at 2.39 percent and 1.29 percent, respectively, are seeing their highest seven-day positivity rate since mid-to-late May.

The state’s rate is largely driven by some of its rural counties, including Cecil County, which borders Delaware and Pennsylvania, and Queen Anne’s County on the Eastern Shore. Cecil County is one of four counties in Maryland with less than 45 percent of its population having received at least one vaccine dose.

Virginia reported 666 new ­cases and two deaths Thursday.

The seven-day average of new daily cases there was 499, compared with 305 one week ago, although the seven-day average of new deaths fell to three, compared with four one week ago.

As officials urge more people to get vaccinated, George Mason University announced Thursday that it is expanding its coronavirus vaccine mandate to include faculty and staff.

The requirement formerly applied only to students without medical or religious exemptions. But in a message to the campus, President Gregory Washington changed course, citing the spread of the delta variant and comments from Centers for Disease Control and Prevention Director Rochelle Walensky, who said the virus is almost exclusively killing unvaccinated people.

“For the sake of all who are unable [to] receive vaccination, the single most effective way to avoid the virus and stop its spread is for the rest of us to get vaccinated as soon as possible,” Washington wrote.

Lauren Lumpkin contributed to this report.