Wildtype SARS-CoV-2 more widespread in 10-19-year-olds prior to vaccine availability in the USA

The coronavirus disease 2019 (COVID-19), which is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has infected more than 187 million people and caused the deaths of over 4 million as of July 13, 2021. Despite the approval and widespread distribution of COVID-19 vaccines in many countries around the world, SARS-CoV-2 continues to mutate. This rise in mutation rates has led to the emergence of several new strains and lineages that are often more transmissible and/or virulent than the original SARS-CoV-2 strain.

Among the several different SARS-CoV-2 strains that are currently circulating, the Delta variant (B.1.672) is estimated to be up to 50% more transmissible than the original SARS-CoV-2 strain. The Delta variant also appears to impact younger groups much more than previous variants that typically spared this patient population from serious infection.

A new study published on the medRxiv* preprint server aimed to investigate the susceptibility of adolescents as compared to older adults in the United States during the summer of 2020.

Study: COVID-19 Due to Wild-Type SARS-CoV-2 More Prevalent in Adolescents and Youth than in Older Adults Based on 19 US States in Fall 2020 Prior to Vaccine Availability. Image Credit: David Tadevosian / Shutterstock.com


At the beginning of the pandemic in March of 2020, COVID-19 was initially reported to affect all individuals, regardless of their age. As more information was discovered on the virus, researchers pointed to higher prevalence rates among older adults, with adolescents being the least susceptible group. To this end, several groups have indicated that the odds of infection are reduced by half in adolescents aged 10-19 years, whereas adults of all ages have comparable susceptibility to COVID-19.

Concerning the United States, however, the current study offers a unique perspective. The data from this study, which was collected before vaccines became available in the United States, show that the prevalence of COVID-19 was, in fact, much higher in adolescents and younger populations as compared to older adults.

The current study

Following up on findings from six states that adolescents were reporting higher cases of COVID-19 as compared to adults in the summer of 2020, the researchers expanded their analysis to 19 American states. Data was collected during the fall of 2020, which is when these states were experiencing surges in COVID-19 cases.

In 16 of the 19 states experiencing these surges in the fall of 2020, the earlier finding was again confirmed that the prevalence of COVID-19 was much higher in individuals between the ages of 10-24 years compared to older adults.

In ten states, the ratio of the number of cases observed to the number of cases expected based on the demographics of the population was significantly greater in patients between the ages of 10 and 19 as compared to older adults. Notably, the three states of South Dakota, Michigan, and Pennsylvania did not follow this same pattern.  

If the age group was shifted to 15-24 years, the four states of Alabama, Florida, Oklahoma, and Rhode Island experienced an increased prevalence of COVID-19 cases as compared to older adults. For instance, in Alabama, the incidence was 5.5% in adolescents and young adults but only 3.5% in older adults. Similarly, the observed cases were 25% to 100% higher in these four states in this age group relative to older adults.

Combining both age groups, adolescents and young adults between the ages of 10 and 24 years in Minnesota and Missouri showed a higher prevalence of COVID-19 and observed-to-expected case ratios than adults over the age of 65.

Wildtype SARS-CoV-2 showed a higher prevalence among 10-19-year-olds

The scientists in the current study did not sequence the strains causing COVID-19 in patients who tested positive for the virus. However, they assumed that these infections were caused by the original SARS-CoV-2 strain. The first SARS-CoV-2 variant to be reported in the United States was the alpha variant, which was first reported at the end of 2020, several months after this study was conducted.

A second important factor of this study is that the infection showed a significantly lower prevalence among older adults than in children, which cannot be accounted for by vaccination of adults.

Possible reasons

One explanation offered here is the increased contact among adults as compared to adolescents. Alternatively, older adults may be more cautious and more adherent to non-pharmaceutical interventions such as mask-wearing and social distancing. Taken together, the researchers believe that both of these social factors had a role in determining the COVID-19 prevalence rates reported in the current study.

As previously mentioned, many studies have reported a much higher prevalence of COVID-19 in older people. The differing results reported in the current study may be due to a combination of factors, including that schools were often closed when these earlier studies were conducted.

Furthermore, early on in the pandemic, there was a general lack of accessibility to testing for the general population. Therefore, adolescents and youth who are mostly asymptomatic or only experience mild symptoms may have evaded the need to get tested before the study was conducted.

In support of this explanation is the fact that less than 2% of recorded infections up to April 2, 2020, were among adolescents. While 60% of pediatric cases were among those aged 10-17 years, the number rose remarkably until there were over four million cases in children under the age of 18 by June 24, 2021. This is comparable to the 2,500 cases that had been reported on April 2, 2020.

Accounting for exceptions

In the current study, an increased prevalence of COVID-19 cases among older people as compared to adolescents was reported in the three states of South Dakota, Michigan, and Pennsylvania. South Dakota, for example, was home to a large motorcycle rally in the town of Sturgis from August 7 to August 16, 2020, with over half a million bikers pouring into town.

The residents of Sturgis, South Dakota generally did not follow mask or social distance guidelines; therefore, a predictable wave of COVID-19 cases followed the event. Most of these cases were among adults, which reversed the trend in this study that the prevalence was higher among 10-19-year-olds compared to older people.

In Michigan and Pennsylvania, election rallies held by former President Donald Trump, again without mask use or social distancing, were the cause of the surges in COVID-19 cases among older adults. While other states also had similar rallies, the number was remarkably higher in these two states, with 15 and 11 rallies, respectively.

What are the implications?

The Real-time Assessment of Community Transmission-1 (REACT-1) United Kingdom study at Imperial College London reported similar trends, with children aged 5-12 and young adults between the ages of 18-24 outnumbering adults aged 65 years or more by fivefold in test positivity. This rise in infection rates has been largely attributed to the Delta variant; however, the authors of the current study disagree.

Our findings indicate that the high prevalence in adolescents and youth is not a novel phenomenon and was present with the wildtype strain.”

In fact, the authors attribute the much bigger difference described in the United Kingdom study to the rollout of vaccines over the past several months that were primarily available only for older adults, thereby reducing their rates of positivity.

The fact that the Delta variant is much more infectious and potentially causes more severe disease, coupled with evidence that COVID-19 is more common among older teenagers and young adults than older adults, should be considered carefully by public health officials who are guiding national policies.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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