Surges in COVID-19 outbreaks may be preceded by an increase of infections in young adults, according to a US study late last week on transmission dynamics by age-group in Morbidity and Mortality Weekly Report (MMWR).
Not only does this add another way to surveil potential spikes, but it adds onus to a population that has been noted to sometimes feel impervious to the worst that COVID-19 can bring—even if that notion is being disproved.
A separate MMWR study based on interviews sheds light on why young adults might engage in COVID-19 risk-taking. And a third study shows that infected young adults can have serious disease, with almost 3% of the more than 3,000 patients studied dying.
A ripple effect from young adults and up
After studying 767 hot spot US counties in June and July, the researchers—part of the Centers for Disease Control and Prevention (CDC) COVID-19 Response Team—found that the percent positivity of SARS-CoV-2 tests for those 0 to 24 years of age began increasing 31 days before the location was flagged as a hot spot.
Comparatively, they say, "Increases in percent positivity among older age groups began after the increases in younger age groups: Among adults aged 25-44 years, 45-64 years, and ≥65 years, increases began 28 days, 23 days, and 20 days, respectively, before hotspot identification."
When the counties became labeled as hot spots, the highest percent positivity was among persons aged 18 to 24 years (14%), followed by ages 0 to 17 (11%) and 25 to 44 (10%). The study also followed caseload numbers for 45 days afterward.
Most age-groups peaked with 10% to 14% infected, but researchers found that those outside the 18-to-24 age-group had slower declining rates. Although most research to date has found that young adults are generally less vulnerable to severe effects from COVID-19, any increased transmissions can reach higher-risk subpopulations.
Young adult behavior in 1 hot spot
According to another MMWR report on Oct 9, the perceived responsibility to keep others safe is one of the main factors that drives young adults to follow public health guidelines such as mask wearing.
"For me it's more of who am I affecting the most," one study participant said in an interview. "When it comes to, like, my grandparents or people at the grocery store, I don't want—even if I do have it, and if I don't have any symptoms, why spread it to other people?"
Still, that sentiment wasn't enough to stop COVID-19 numbers from ballooning in Winnebago County, Wisconsin, where that participant was from—or perhaps for some, the realization came too late. While the CDC interviews took place in July, in early May, the 172,000-person county already had the 19th-highest cases per capita in the nation, according to the Rockford Register Star.
From Mar 1 and Jul 18, 240 young adults tested positive, accounting for 32% of the county's cases, the MMWR study noted.
Data pulled from Wisconsin's Electronic Disease Surveillance System on those 240 cases showed that 16% of young adults were asymptomatic. During the 14 days before developing symptoms or receiving a positive test, 38% said they attended a social gathering and 83% of the 72% who were employed were working outside their home.
To better understand the spread of the virus, the CDC conducted 13 interviews among young adults in the county and found that while they said they were limiting social gatherings, they would still meet up for events like bonfires or to go to bars.
Inconsistent information, misinformation, peer pressure, a feeling of security among friends, and unenforced or nonexistent health mandates also deteriorated guideline adherence. In other instances, safety concerns were overridden by the belief that their individual health would not suffer severe consequences.
These insights cannot be generalized to young adults across the country owing to the recruitment method (snowball sampling), self-reporting bias, and the homogenous location, but the researchers—from the CDC and Wisconsin—hope they can improve public health strategic communications.
COVID-19 harms young adults, too
Most coverage on COVID-19's severe outcomes has centered on the elderly, but a research letter published last month in JAMA Internal Medicine reveals that it may be more dangerous for hospitalized young adults than many realize.
Of 3,222 adults aged 18 to 34 who were hospitalized for COVID-19, more than 1 in 5 (21%) needed intensive care, 333 (10%) required mechanical ventilation, and 88 (2.7%) died. And, although the latter statistic may seem low, the researchers point out that it's approximately double the mortality rate if the same group were to be hospitalized for a heart attack.
"We know nothing about the total denominator of patients who got an infection," said corresponding author Scott Solomon, MD, in a press release. "We think the vast majority of people in this age range have self-limited disease and don't require hospitalization. But if you do, the risks are really substantial."
Similar to findings in other populations, obesity or morbid obesity, diabetes, and high blood pressure seemed to escalate COVID-19 symptoms. For instance, hospitalized patients who were morbidly obese made up 41% of those who died or needed mechanical ventilation.
Overall, the study found that if young adults had two or more of these comorbidities, they had the same risk for adverse outcomes as those 35 to 64 years old. Diabetes appeared to be associated with increased risk, but there was no statistical significance.
The median length of hospitalization was 4 days. The researchers noted that there was a disproportionate amount of minorities admitted (57% were black or Hispanic), but the odds of death or mechanical ventilation did not vary significantly across race. All cases in the study were between Apr 1 and Jun 30 and found via the Premier Healthcare Database, which pulls information from 1,030 US hospitals and healthcare systems.
Addressing outbreaks in hot spots
All three of these studies point to the importance of controlling the spread of SARS-CoV-2 among young adults, whether it relates most to case network, failures in public health practices, or the young adults' own health.
As the researchers from the first MMWR report conclude, "There is an urgent need to address transmission among young adult populations, especially given recent increases in COVID-19 incidence among young adults. These data also demonstrate the urgency of health care preparedness in hotspot counties, which are likely to experience increases in COVID-19 cases and hospitalizations among older populations in the weeks after meeting hotspot criteria."