Analysis: Northern Virginia experiencing slight spike in COVID-19

Analysis: Northern Virginia experiencing slight spike in COVID-19
An illustration of the COVID-19 virus. (Image/Centers for Disease Control and Prevention)

By Denise Dunbar |

The seven-day, moving-average positivity rate for COVID-19 has become a relied-on indicator nationwide for how states, regions and cities are faring in the coronavirus pandemic.

This average forms the basis for determining which states are considered “hot spots.” Travelers from hot spot states often are required to quarantine for up to 14 days upon entering states with much lower COVID-19 positivity rates.

Early on, before test kits for COVID-19 were widely available, positivity rates were extremely high, as only the sickest patients with clear symptoms were being tested. For instance, on April 21, the seven-day average positivity rate statewide in Virginia was 20.6% – but only 2,753 tests were conducted that day throughout the commonwealth.

Seven-day moving averages of positive COVID-19 tests on particular dates by locality.

Exactly one month later, on May 21, the statewide positivity rate had fallen to 13.8% and the number of tests administered statewide had risen to 8,385. It wasn’t until early June that Virginia began consistently reaching Gov. Ralph Northam’s goal of more than 10,000 tests per day.

On Aug. 21, the most recent date for which seven-day moving average data was available on the Virginia Department of Health website, Virginia’s statewide COVID-19 positivity rate was 6.4% – which was just slightly higher than the City of Alexandria’s rate of 6.2%.

This article looks at Alexandria’s seven-day average positivity rate going back two months, and a comparison of how the city’s COVID-19 positivity rate stacks up with those of neighboring Northern Virginia localities and with cities in other regions of Virginia. The data comparison points are exactly two months apart: Aug. 21, July 21 and June 21, and all data comes from the VDH website.

While these data points show the COVID-19 positivity rate moving directly up or down, in reality the data fluctuated considerably during the intervening month, and these arbitrary one-month intervals capture the general trends.

Two months ago, Alexandria had the second-lowest moving-average positivity rate in Northern Virginia: 5.1% compared with Arlington’s 3.7%. The city’s rate on June 21 bested that of Fairfax County, which was 7%; it was also lower than the rate in Loudon County, 7.9%, and about half of Prince William County’s rate of 10.1%.

One month later, by July 21, the positivity rates of all five jurisdictions had converged, and only 3.3% separated the locality with the lowest positivity rate, Arlington at 4%, from the highest, Prince William with 7.3%.

Seven-day moving averages of positive COVID-19 tests on particular dates by locality.

The positivity rates in Alexandria and Arlington followed the same general path of starting lower than their Northern Virginia counterparts, but increasing slightly during each of the two months. Alexandria’s seven-day COVID-19 positivity rate rose from 5.1% on June 21 to 5.4% on July 21 to the current 6.2%. In Arlington, the upward trend has been slightly more pronounced, rising from 3.7% to 4% to 5%.

The data show that COVID-19 positivity has risen across Northern Virginia in the past month, as all five localities had higher seven-day average positivity rates on Aug. 21 than they did on July 21.

While two months ago Alexandria had the second lowest seven-day moving average positivity rate in Northern Virginia, by Aug. 21 it had the second highest – topped only by Prince William County’s rate of 8.3%.

Comparing Alexandria’s seven-day average COVID-19 positivity rate with cities in four different regions of the state – Norfolk, Pittsylvania-Danville, Richmond and Roanoke – reveals some similar trends and some that are quite different.

As in Northern Virginia, Alexandria’s position two months ago relative to these cities in disparate parts of the state was better than its current standing.

On June 21, Alexandria had the lowest average COVID-19 positivity rate of the five cities – though the spread was narrow, ranging from Alexandria’s 5.1% to that of Roanoke and Pittsylvania-Danville, which both had rates of 7.2%. But by Aug. 21 Alexandria was only third lowest, having been surpassed by both Richmond and Roanoke.

By July 21, the positivity average had spiked sharply in Norfolk and Pittsylvania-Danville, to 13.8% and 14.6% respectively. It had risen slightly in Alexandria and Richmond and had declined in Roanoke.

Roanoke had the opposite pattern of Alexandria, experiencing slight declines in its seven-day average positivity rate during the two-month period, going from 7.2% on June 21 to 6.4% on July 21 to 5.4% on Aug. 21.

Alexandria is the only one of these five cities around the state to experience an increase in its seven-day average COVID-19 positivity rate between July 21 and Aug. 21.

The data overall suggest that by June 21, inside-the-beltway locations such as Alexandria and Arlington had rebounded from the initial wave of COVID-19, which struck the Northeast before gaining widespread traction in Southern states.

By July 21, that second wave was peaking in the South, and Virginia’s southern-most and coastal regions were hardest hit. By Aug. 21, that torrent had subsided slightly in the southern and eastern regions of the state, but a subtle upward trend in positive cases is occurring in Northern Virginia.

It appears that Alexandria’s positivity rate may continue to nudge upward, as the city experienced 39 new positive cases on Tuesday, which was the highest single-day case total in almost a month.* In fact, it was the second-highest single-day positive total in Alexandria since May 29. 

*A single-day case total is just that, the number of cases reported on a given day. This is not to be confused with the seven-day positivity moving average, which captures the trend over the previous week and is a factor of how many tests were administered.