All jurisdictions in the D.C. region are now reporting substantial transmission of COVID-19, underscoring the highly contagious nature of the delta variant as local officials rush to stem the tide of another surge.

The District, most of Northern Virginia, Prince George’s County, and as of Thursday, Montgomery County, are all reporting substantial community spread of the virus, according to their own metrics or the Centers for Disease Control and Prevention’s transmission tracker. According to the federal health agency’s metrics, “substantial” transmission is any locality that reported 50-100 new cases per 100,000 residents over the past week, or is reporting a positivity rate between 8% and 10%.

The CDC issued new guidance last week, recommending that all individuals — regardless of vaccination status — resume wearing masks indoors in parts of the country where virus transmission is “high” or “substantial.” Local health officials, in turn, have renewed their push to vaccinate more residents and reintroduced indoor mask mandates to slow the spread.

D.C. met the substantial mark last Wednesday, prompting Mayor Muriel Bowser to reinstate the city’s indoor mask mandate on Saturday, July 31. The District was the first jurisdiction in the region to require masks in all indoor settings since dropping the mandate earlier in the spring.

Bowser stopped short of hinting at further restrictions during a press conference announcing the mandate. She hoped, instead, that the indoor mask mandate “stops [the case increase] before it goes too far.”

As of Aug. 5, D.C. is reporting a seven-day average of 84 new cases a day — the highest that metric has been since April, and a nearly eight-fold increase since the beginning of July. The city’s average daily case rate per 100,000 residents is also at its highest point since April.

The city’s average death rate is low and staying low, and hospitalizations remain fairly stable after steadily decreasing throughout the spring. However, similar to other case surges over the past 15 months, any increase in hospitalizations and deaths lags a few weeks behind any noticeable increase in infections.

“Hospitalization lags cases by about two weeks, and death by about three weeks…so look 21 days out from where we are today, and you’ll see a commensurate spike in deaths,” according to Neil J. Sehgal, a public health professor at the University of Maryland. “It’s going to be a lot lower than it was before vaccines were available, because we did do a really good job of vaccinating the elderly and protecting many of the vulnerable, but that protection is not total.”

About 54% of D.C.’s total population has been fully vaccinated, according to the city’s data, and infections in vaccinated residents have been rare. Of the total infections recorded since January, fully vaccinated individuals make up about 1% of those cases, per the city’s breakthrough dashboard. (The dashboard has not been updated since July 11). Bowser and D.C. Health officials are continuing vaccine outreach programs, especially in wards 7 and 8 where rates have lagged for months behind the city’s whiter and wealthier wards. The city has also announced a slew of vaccine incentives for kids ages 12-17, hoping to increase vaccination rates before students return to classrooms this fall.

In Northern Virginia, five local departments — Alexandria City, Arlington, Fairfax, Loudoun, and Prince William counties — issued a joint statement on Aug. 3, recommending indoor mask wearing for all individuals. All five jurisdictions are currently reporting substantial community spread, with Alexandria and Loudoun County among the first to hit that mark last week. Virginia Gov. Ralph Northam tweeted out a recommendation for indoor mask wearing last week, but has so far declined to reinstate a mask mandate for the commonwealth.

The average daily case count in Northern Virginia now tops 180, a number last reported in early May, according to the state’s COVID-19 dashboard. Hospitalizations, which increased slightly near the end of July, have decreased again in the region; deaths remain at the lowest average daily counts in the pandemic.

While the low metrics for deaths in the region demonstrate the efficacy of vaccines (fully vaccinated individuals make up less than 1% of the state’s total infections), Sehgal says that a sizable increase in infection across an area is still cause for concern, given that a little less than half of Virginia’s population is fully vaccinated.

“One of the things that vaccines did was they decoupled infections from hospitalizations,” Sehgal says, while pointing out that it is possible for fully vaccinated individuals, in rare occasions, to get seriously ill from the virus. “Half of the state, essentially, is still subject to serious illness, hospitalization and ultimately death from infection, and so what [that decoupling] means is it’s taken [the vaccinated] half of the population out of the equation. But there’s still a serious risk of the other half.”

In Maryland, the D.C. suburbs of Prince George’s and Montgomery counties moved into the substantial spread zone over the past week. Maryland, like D.C. and Virginia, has fully vaccinated about 54% of the total population,

In Prince George’s County, the daily case rate per 100,000 residents is now over 11, a sharp increase from June, when that metric was nearly 1. The county is now in the substantial transmission zone, per the CDC. Montgomery County has seen a similar increase, reporting an average daily case rate of 8 as of Aug. 4, compared to 0.5 in June. While the county has not officially moved into the CDC’s measure for substantial spread, the jurisdiction’s own more recent data reflects substantial transmission of the virus in the community. Both Montgomery County and Prince George’s County moved to reinstate an indoor mask mandate for all residents on Thursday.

“With our highly-vaccinated population, these kinds of precautions can, hopefully, make a difference and keep us from falling into a place that we really do not want to be in,” Montgomery County Executive Marc Elrich said during a media briefing Wednesday, referring to a forthcoming mask mandate. (Currently, Montgomery County boasts a vaccination rate above the statewide rate, with 71% of the county fully vaccinated, according to the CDC.)

While Sehgal says it’s unlikely that either Maryland or Virginia would reinstate a statewide mask mandate, he says that masking, in addition to vaccines, is necessary to stop the virus – especially the more-transmissible delta variant — from rapidly spreading, resulting in more hospitalizations and deaths. (During a press conference Thursday afternoon, Maryland Gov. Larry Hogan confirmed that he will not impose a new statewide mask mandate, but both he and Virginia Gov. Ralph Northam handed down vaccine requirements for some state employees – with exceptions – on Thursday.)

“We didn’t stop wearing seatbelts when cars got airbags,” Sehgal says. “Layered prevention works, and no one barrier is foolproof.”

This story has been updated to reflect Montgomery County’s vaccination rate using CDC data, and to reflect that Prince George’s and Montgomery counties reintroduced mask mandates.