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The SARS-CoV-2 Delta Variant: Where Are We Now?

by | Sep 15, 2021

David Wessner, Davidson College Professor of Biology


As we all have learned over the past 18 months, pandemic conditions can change quickly. The rapid spread of the SARS-CoV-2 Delta variant, also referred to as B.1.617.2, certainly confirms that statement. In June, experts agreed that Delta would become the dominant strain in the United States later this summer. According to the CDC, Delta achieved that status well before the 4th of July. Today, Delta accounts for over 98% of new SARS-CoV-2 infections in the U.S. Moreover, the number of new cases of COVID-19 has been increasing throughout the United States. According to the CDC, approximately 40,000 new cases per day were being reported a year ago. Today? Over 140,000 cases per day were reported in late August. COVID-19 deaths also have risen, with approximately 200 deaths per day reported in early July and roughly 1,200 deaths per day reported in early September.

Although all states have seen an increased number of COVID-19 cases and deaths since June, the impact has differed from state to state. In some states, like California and New York, the increases have been relatively modest. In other states, like Florida and West Virginia, the current number of daily cases exceeds the number of cases seen in those states during the winter surge. Here in North Carolina, the number of daily cases has increased from roughly 400 in late June to over 5,000 in late August. Within the past couple weeks, new cases and COVID-related deaths seems to be plateauing or even decreasing here and in many other states. However, the numbers remain troublingly high. Clearly, the pandemic has not ended.

So, what do we now know about the Delta variant?

Like the other SARS-CoV-2 variants, Delta possesses a series of mutations that differentiate it from other strains of the virus. Currently, researchers are most interested in mutations observed in the gene that codes for the spike protein, the molecule on the virus’s surface that allows it to attach to our cells and begin the infection process. The mutations present in Delta seem to help the virus enter our cells and produce more copies of itself more rapidly. The result? Studies have shown that people infected with Delta may produce 1,000 times more virus than people infected with the earlier SARS-CoV-2 variants. With more virus being produced, the likelihood of transmission increases. Indeed, Delta is thought to be 40% more transmissible than the Alpha variant, and Alpha was significantly more transmissible than the original strain of the virus.

What about the virulence of Delta?

Most typically, the term “virulence” refers to the severity of disease associated with an infectious agent. Throughout the COVID-19 pandemic, measurements of symptomatic infections, hospitalizations, and deaths primarily have been used to evaluate the virulence of different strains. In a report recently published in The Lancet, researchers in Scotland concluded that individuals infected with the Delta variant were twice as likely as individuals infected with the Alpha variant to be hospitalized. A preliminary study by researchers in Canada showed that infections with the Delta variant led to higher rates of hospitalizations, ICU admissions, and death. These data certainly are not conclusive, but they do suggest that Delta may be more virulent than its predecessors. However, it’s worth noting that the majority of hospitalizations and deaths associated with Delta have been in unvaccinated people.

Do the vaccines still work?

This question, perhaps more than any other, weighs heavily on the minds of many of us. Are the current vaccines less effective at preventing infections caused by Delta? Yes. But, the vaccines still are very effective. Let’s look at some of the preliminary data. In a study just published in Morbidity and Mortality Weekly Report, researchers analyzed the vaccination status of nearly 33,000 individuals who were hospitalized or visited urgent care centers with COVID-19 during June, July, and August 2021. Because of this timing, it was assumed that most of these people were infected with the Delta variant. Based on their analysis, overall vaccine effectiveness (VE) was estimated to be 86%. This value differed for the three vaccines available in the United States, with an estimated VE of 92% for the Moderna vaccine, 77% for the Pfizer/BioNTech vaccine, and 65% for the J&J/Janssen vaccine. These numbers may reflect the ability of Delta to evade vaccine-induced immunity. The numbers also may reflect a general waning of vaccine-induced immunity that occurs over time. Importantly, even though these numbers are lower than the VE numbers initially reported for these vaccines, they still indicate that all three vaccines are very effective at preventing severe disease.

What comes next?

The Delta variant is widespread. It’s highly transmissible. It may be more virulent than other variants. It’s probably blocked less well by the vaccines. That’s the bad news. Here’s the good news. The vaccines are still very effective. Basic nonpharmaceutical interventions – wearing a mask, remaining physically distanced from others, avoiding large indoor gatherings – still work. By getting vaccinated and continuing to use these basic mitigation strategies, each of us can minimize our own risk of becoming infected and, if we do become infected, of transmitting Delta to someone else. As I noted before, the pandemic isn’t over. Far from it. And Delta certainly won’t be the last variant that we encounter. Indeed, scientists currently are monitoring a new variant of interest, Mu. However, we have the tools to lessen the effects of Delta, Mu, and the future variants of SARS-CoV-2 that inevitably will emerge.

David Wessner

David Wessner is a professor of biology at Davidson College, where he teaches classes on microbiology and HIV/AIDS. Before joining the faculty at Davidson, he researched coronavirus pathogenesis at the Navy Medical Center. He has co-authored an undergraduate microbiology textbook and The Cartoon Guide to Biology.

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