Even unvaccinated residents of the Chattanooga region are experiencing reduced COVID-19 transmission thanks primarily to the vaccination of others. At the same time, they remain at higher risk for illness and contribute to the continued spread of the coronavirus, according to an analysis of county-level COVID-19 case and vaccination data.
In an effort to better understand how COVID-19 is impacting the region's unvaccinated population, the Times Free Press calculated the seven-day average new daily case and new daily hospitalization rates for the 19 Tennessee and Georgia counties that comprise the Chattanooga region, home to roughly 1.2 million people.
The review covered early January — around the time that priority groups began receiving coronavirus vaccines — through the end of May, using publicly available state data.
Those rates were then recalculated by subtracting the portion of the population that had received at least one dose of COVID-19 vaccine, since the vaccinated population's risk of contracting the coronavirus and being hospitalized is low.
To account for "breakthrough cases," which are rare in fully vaccinated people and sometimes occur in partially vaccinated people, only 85% of the region's vaccinated population, which amounted to 318,540 people as of May 31, was subtracted from the total population to form the "denominator" of the rate calculations.
Using rates to compare health data:
To calculate a county’s COVID-19 incidence rate or vaccine coverage, the number of COVID-19 cases or vaccinations — the numerator — is divided by the number of people in the population — the denominator — and then multiplied by 100,000 to make the rate comparable with other counties.
Rates put data into perspective of the size of the population, so they can be used to compare data from different locations, at different times or among different groups.
The results show that although COVID-19 cases and hospitalizations continue to trend down across the region, that trend is slightly less pronounced after removing the vaccinated population from the equation. The COVID-19 case rate among the estimated unvaccinated population at the end of May mirrored the case rate of the general population a week earlier, according to the analysis. Hospitalization rates paint a similar picture.
Dr. William Schaffner, professor of medicine in the infectious diseases division at the Vanderbilt University School of Medicine, said the Times Free Press calculations — which were modeled after a similar state-by-state analysis conducted last month by The Washington Post — help to emphasize that those who have yet to be vaccinated remain most at risk for illness and are the primary drivers of the region's ongoing COVID-19 outbreak.
"Clearly, the vast majority of people who are finding their way into the hospital today with COVID infection are people who are unvaccinated," Schaffner said. "The whole purpose of the vaccine is to keep us out of the hospital, and you can imagine that the number of people in the hospital would really plummet even further with COVID if the rest of the population were vaccinated."
Tennessee and Georgia remain among the least vaccinated states in the country, with 39.5% and 40.5% of their populations, respectively, at least partially vaccinated, according to data from the U.S. Centers for Disease Control and Prevention. Those rates are below the national average of 51% at least partially vaccinated and the rates in the most vaccinated states — Vermont, Hawaii and Massachusetts, which have managed to give at least one dose of COVID-19 vaccine to 71%, 67% and 67% of their populations, respectively.
In the Chattanooga region, Meigs County, Tennessee, is the most vaccinated county in the region per capita, with nearly 50% of the population having received at least one dose as of Friday. Grundy County has the lowest vaccination rate, with only 20% of the population having at least one dose.
Despite the Chattanooga region's low vaccine coverage, Shaffner said even a modest number of protected individuals is helping to lower transmission rates, and other local factors are likely helping drive COVID-19 numbers down.
"We are gradually accumulating — albeit less than in many other states — vaccinated people, and so we're getting some partial community protection," Schaffner said, adding that other factors contributing to the decline in cases include those who have gained immunity through prior infection, a decrease in testing and the nature of disease transmission in rural communities.
Other factors that can affect transmission are weather, if schools are in session, local regulations or policies and whether more contagious variants are circulating.
Case and hospitalization rate differences between the unvaccinated and total population were greatest in Hamilton County, because it's the most populated and has the second-highest rate of vaccination coverage in the region, but much less evident in small, rural counties. Several rural counties had multiple days during which no new cases or vaccinations were reported.
"Because the population density is less, people's interaction is less. But in those circumstances, the virus is going to continue to smolder," Schaffner said.
Samuel Scarpino, an assistant professor in the Network Science Institute at Northeastern University and director of the Emergent Epidemics Lab, has conducted data analysis and mathematical modeling of COVID-19 outbreaks in communities across the country, including in rural counties. Although the isolated nature of rural counties gives them some protection from the disease, their "tight knit" social patterns also make them highly susceptible once it arrives, he said.
"Maybe there's one church and everybody goes to the same church, or there's one grocery store and everybody goes to the same grocery store," Scarpino said. "[COVID-19] goes from zero to a huge problem way faster than it often does in a city. And in the U.S., we have a lot lower coverage per capita in terms of hospitals in rural areas, so you can run out of hospital beds fast."
Scarpino said most of the available data and attention around vaccine coverage is focused at the state level, but that obscures what's happening in localities.
"Really, what's going on is that there are some communities that are way below the average and some communities that are way above the average," he said, citing an example from Massachusetts, where 67% of the state is at least partially vaccinated.
However, in one town, nearly 100% of the residents over age 17 have been vaccinated, while in another around 35% of adults have been vaccinated, Scarpino said.
"What that means is that in those towns where you're below 50-60% of adults, you're going to be at much higher risk with these new variants for local surges," he said.
Another factor to consider when it comes to COVID-19 risk for the unvaccinated is whether a community has already experienced severe outbreaks.
Maine is an example of a state with high vaccination coverage (64% of the population has received at least one dose) that is still in the top quarter of states in terms of new COVID-19 cases per capita, which Scarpino said is likely because Maine never experienced a large COVID-19 surge and has just begun to experience warmer weather.
"A community that already had a big outbreak and has 30-40% vaccination coverage, maybe effectively, the immunity is 50, 60, 70%, whereas another community that really didn't have much of an outbreak and has 40% vaccination coverage may actually be at risk for another surge," he said. "If there's one message, it's really that your local community is what matters, and if you haven't had a big COVID outbreak, and you've got low vaccination coverage, you're going to be at high risk."
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The Times Free Press created the adjusted rates for COVID-19 cases and hospitalizations in Southeast Tennessee and Northwest Georgia using data from the Tennessee Department of Health and the Georgia Department of Public Health. The analysis included Bledsoe, Bradley, Coffee, Franklin, Grundy, Hamilton, Marion, Meigs, McMinn, Polk, Rhea and Sequatchie counties in Tennessee; and Catoosa, Chattooga, Dade, Gordon, Murray, Walker and Whitfield counties in Georgia.
New cases and hospitalizations were measured with a seven-day moving average. The total vaccinated population was gathered using state reports on the number of vaccine recipients in each county, signifying the number of people who had received at least one dose of the vaccine. County-level recipient data was first made available on Jan. 5, 2021.
To calculate the total county population that is immune to the virus, the Times Free Press used a similar methodology to that of The Washington Post. Some people who are vaccinated can still be infected with the virus, known as breakthrough cases, though those infections are typically mild. To account for these instances, the Times Free Press used 85% of the total number of vaccine recipients each day to create the “vaccinated population” in each county.
Subtracting the vaccinated population in each county from the total county population, gathered using data from the U.S. Census Bureau, provided a population that is still at risk of contracting COVID-19. This population, signified as the “unvaccinated population,” was then used to measure case rates and hospitalization rates for each county.
The Times Free Press did not include people with prior coronavirus infections who are now recovered in the “vaccinated population,” because those same people may have also received vaccines. People who recover from COVID-19 develop a level of natural immunity, but it’s unclear how long that immunity lasts.
Due to the lack of available data and unknowns surrounding breakthrough cases, people with prior infections who’ve been vaccinated, variants, and many other social, environmental and health factors that affect disease transmission and data collection, these calculations should only be viewed as estimates of how COVID-19 is spreading in the region’s unvaccinated population.
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