Update: A week after this story was published, Dallas County removed the herd immunity dashboard from its website and is no longer reporting that the county has achieved herd immunity. A statement on the county's website, which can be read here, said the delta variant was not factored into the initial herd immunity models and that, "we will know when we have reached the point of herd immunity when new COVID-19 cases are no longer appearing." The rest of the original report appears below.
According to county health officials, Dallas County has obtained herd immunity to the COVID-19 virus. But what does reaching that goal mean?
According to the Centers for Disease Control and Prevention, herd immunity, or community immunity, is "a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely." When herd immunity is present, unvaccinated people (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community.
Hitting herd immunity in Dallas County meant 80% of the population either had been vaccinated against the virus or had been already infected with the virus and therefore had antibodies to prevent subsequent infection.
Get DFW local news, weather forecasts and entertainment stories to your inbox. Sign up for NBC DFW newsletters.
In February, the Parkland Center for Clinical Innovation (PCCI) originally forecast herd immunity would be reached in mid-June. However, a slowdown in COVID-19 vaccinations forced the forecast to be revised to late July. PCCI and DCHHS debuted a dashboard this week to track herd immunity.
The milestone was reached this week, a little earlier than the latest forecast. As of Wednesday, July 7, the Dallas County Health and Human Services/PCCI dashboard said 80.4% of the population now meets the criteria of either having been infected or having been vaccinated and, thusly, the county has reached herd immunity.
Full coverage of the COVID-19 outbreak and how it impacts you
It is important to remember that while herd immunity may provide more protection for a community, it is not a guarantee that people will not continue to be infected with COVID-19.
Dr. Joseph Chang, Chief Medical Officer at Parkland, told NBC 5 Wednesday afternoon that herd immunity essentially means 8 out of 10 people in the county shouldn't be able to get coronavirus.
Chang went on to say, however, that some age groups are more protected than others. Older groups have about 90% herd immunity while some younger, more vaccine-resistant groups, may only be 40% protected. Children under 12, who are not yet able to get vaccinated, remain at risk for infection.
PCCI CEO Steve Miff said in a statement Wednesday that while herd immunity "represents good progress" in the community, "it is important that we understand the work is not over."
"While the whole community on average reached the 80% mark, there are only 49 ZIP codes above the 80% threshold with 45 ZIP codes still below the 80% mark," said Miff in a statement Wednesday. "There are still significant pockets in the community that remain vulnerable."
Experts like Chang and Miff still encourage those who are not vaccinated against the virus to get a vaccine as soon as possible.
"These vaccines are extremely effective even against all of these brand new variants that are coming out, whether it's the delta or new Lambda that everybody is talking about," Chang said. "We know that these vaccines from Pfizer and Moderna are very effective in preventing hospitalizations and death and that is, of course, the most important thing."
Chang said patients hospitalized at Parkland for COVID-19 are now almost exclusively younger people and that all of them are unvaccinated.
"The delta variant is here, it is getting people sick and do you know what? It is those 30 and 40-year-olds that are getting sick," Chang said. "Right now, in my hospital, if you took a look all the way back to February, all of the COVID admissions, none of the people who have been admitted to my hospital with COVID have been vaccinated. That is the power of vaccination."
How is Herd Immunity Calculated
DCHHS calculates herd immunity using the number of confirmed, probable and projected COVID-19 cases plus the number of people who have received at least one dose of the vaccine, minus an estimated overlap.
The county health department dashboard on Wednesday showed 1.21 million people in the county had received at least one dose of the vaccine (44.19%) and that 989,000 people had completed a full cycle. The DCHHS dashboard also showed there were 306,000 confirmed and probable cases of the virus since March 2020.
PCCI said the calculations used to measure heard immunity track individual-level data for both vaccinations administered and COVID test results since the beginning of the pandemic. For those infected, yet not tested there are a 4x Adjusted Incidence Rate Ratio [AIRR] for the adult population and 5x for the pediatric group based on national and local seroprevalence data. The model also calculates an overlap of 28% of the vaccinated population of Dallas estimated to have had prior COVID-19 infection and recovered.
PCCI said the current delta variant is predicted to make up about 25% of COVID-19 cases locally, doubling approximately every two weeks. In one month, PCCI said, that could put the delta variant in the range that has caused a new wave in infections in the UK, though their estimated immunity was below the herd immunity threshold for delta.
The significantly higher viral loads and more infectious nature of the delta variant could put the herd immunity target as high as 88% to suppress infection spikes when the delta variant becomes the dominant variant in a few weeks' time.
PCCI said the message is simple, don't wait to get vaccinated.
"For those still hesitant, the safety and efficacy studies to date are overwhelmingly positive. There are also two key upcoming milestones that should give further confidence to those who remain hesitant: Full FDA approvals for the mRNA vaccines expected in the upcoming weeks and approval for the under 12-year-old groups in the fall," Miff said in a statement.