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Man says he's lost family to virus. Why he's not wearing a mask.
03:29 - Source: CNN

Editor’s Note: This is the second installment in a series of regular columns on making choices in the era of Covid-19 by Erin Bromage, an associate professor of biology at the University of Massachusetts Dartmouth. His research focuses on the evolution of the immune system and how animals defend themselves from infection. Follow him on Twitter @ErinBromage. The views expressed are his own. View more opinion articles on CNN.

CNN  — 

In the past year, I watched as Australia was ravaged by bush fires. Between episodes of raging walls of fire, there would be weeks of calm, where the fires smoldered through sparse bushland, only to erupt into towering infernos again when the flames reached tinder-dry, old-growth forests.

This ebb and flow of the smoldering threat followed by devastating blazes repeated throughout the Australian spring and summer.

Erin Bromage

We are seeing a similar cycle of tragedy unfold in the US. SARS-CoV2 infections started as an unseen threat, with pockets of infection smoldering in our communities early in the calendar year. It wasn’t until an ember hit a susceptible population in nursing homes did we start to appreciate the severity of the threat we faced.

At the start of the pandemic, when we were all so unprepared, our focus was on the mounting infections and the deaths in our older and less healthy population. We witnessed the same phenomena in Italy and Spain. Yes, people aged 40-60 years were being infected and becoming sick, and many were hospitalized, but the ICU rooms at hospitals were predominantly filled with people of older ages and comorbidities

As the pandemic ramped up, we looked in awe at the response of South Korea. Infections accelerated at a similar pace to the US, but there was significantly lower mortality experienced in that country.

At the time, some speculated that the differences in mortality were due to differences in the age demographics of the people infected.

In South Korea, with extensive early testing and an enviable contact tracing program, more than 75% of infections were in people under 60 years old, and about 45% of infections were in people under the age of 40.

In contrast, the US initially had the resources to test only the sickest of patients and because of this limitation in testing capacity, data skewed heavily toward those who were older and more likely to be hospitalized. The rate of viral infections in the elderly caught fire and blazed, and in the effort to contain and stop this unfolding tragedy, we took our eyes off what was happening to the younger people. What was and is their role in this pandemic?

The role of the young and healthy in this pandemic is beginning to reveal itself.

The 20- to 40-year-olds appear to be spreading the infection unperceived. They are just as easily infected as the elderly, but much more likely to show no or mild symptoms. People in these age groups are the ones who have allowed the virus to smolder through our communities and erupt into flames when they make contact with a susceptible population.

Unlike the older populations, where the fraction of tests that are positive have decreased markedly over time – likely evidence that we are doing better at protecting vulnerable people – when we look at the 18 to 49-year-olds, we see that the number of positive cases has remained more or less constant throughout time.

We are now seeing that more than 60% of all infections in the US are occurring in people under the age of 50.

The skewing of the infection rate toward this younger age group, those less likely to have severe symptoms and outcomes, could explain why we are seeing a nationwide reduction in hospitalizations and death.

But the emerging data about the infection rate for those under 50 years old is revealing that the 20- to 40-year-old segment of our population may in fact be the force driving this pandemic.

A recent contact tracing study performed in Japan demonstrated how significant 20 to 40-year-olds are in the initiation of new clusters of infection. About 50% of all clusters traced – outbreaks in which at least five new people were infected – were initiated by this age group. A significant revelation from this research was that the majority of the 20-40-year-old index cases were showing no disease symptoms at the time of contact with the people they infected.

Eighty-one percent of all new virus transmissions, resulting in outbreak clusters, happened in the days leading up to, or on the day of symptom onset. So, these individuals were unwittingly infecting others before they experienced any symptoms of the disease themselves.

Other data shows that these infected younger folks initiated outbreaks in bars, restaurants, gymnasiums and concerts. This is of no surprise to anyone following the data, as this is a pattern we have seen repeated in South Korea and are now observing in the US.

One of the eye-opening outbreaks described in this tracing study was as at a live music concert where a single person’s case resulted in a performer, event staff and members of the audience becoming infected. In total, more than 30 people were infected at the concert.

Knowing that younger people are just as easy to infect, but show fewer and milder symptoms than those who are over 50, allows the virus to slowly burn in the background of our communities.

We see this trend play out in the testing results. However, if the virus continues to burn through this age group, with their increased mobility and their importance within the essential workforce, it will only be a matter of time before we see their infections spark an inferno of sickness and death in vulnerable populations.

For months, I have been particularly troubled by the data in Florida. Florida seemed to have dodged a bullet after spring break revelers in March, appeared to reopen safely in May and the Covid-19 mortality rate has stayed curiously low.

But in the background, the demographics of new infections were changing. The median age of infected people dropped from 65 in March to 35 years old in June. We are now seeing a substantial increase in new daily cases in that state – a 300% increase in the past two weeks, which cannot be attributed to greater testing. We are likely seeing the rise in new infections because of the increased mobility and large social networks of younger people.

While the rate of infection stays highest in this younger population, the burden on the health care system remains low. We know there is a large pool of susceptible people in Florida. We are seeing troubling signs emerge in Miami-Dade as admitted patients approach the April peak, but statewide the hospitalization numbers are flat.

I fear it is only a matter of time, however, before the virus finds its way into a household or a workplace where people more susceptible to poorer outcomes from infection can be found.

A similar pattern is emerging in Arizona. Forty-seven percent of all cases are in the 20-44 age group. However, unlike Florida, the virus has now made its way into vulnerable populations. We are seeing the number of patients admitted to the hospital with Covid-19 rise rapidly, and a concerning increase in the number of people on ventilators.

The embers from those young people have indeed found their way onto kindling and have started a fire.

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    We need to start paying more attention to the role of the sub-40 age group in viral transmission. And we need to do this soon. Their high mobility and complex social and employment networks provide the perfect opportunity for the virus to continue to smolder through our communities.

    We need to focus more effort in fully characterizing their role in asymptomatic and pre-symptomatic infection transmission chains. One of the many reasons this is especially important is that universities are going to reopen in varying degrees of capacity in the fall, which will place the embers of youthful infections in classrooms with the tinder of age – and I don’t see that ending well.