A COVID-19 Update: The Delta Variant
In the United States, life seems to be returning to some semblance of pre-pandemic normalcy. Restaurants, theaters, and sports venues are open and mask requirements have been lifted in many places. The daily averages of new cases and deaths have dropped to their lowest levels since late March, 2020. To a large extent, these decreases have resulted from the mitigation efforts practiced so faithfully by so many of us and the remarkable efficacy of the available vaccines. However, the pandemic is not over. Indeed, the SARS-CoV-2 Delta variant may reverse some of our hard-earned achievements.
The Delta variant, also referred to as B.1.617.2 was first identified in India in December, 2020, and has quickly become the most prevalent strain of SARS-CoV-2 in many parts of the world. In the United Kingdom, for example, infections with Delta were first reported in February, 2021. Today, over 90% of new infections are caused by this variant. And the daily number of new cases in that country has skyrocketed. About six weeks ago, the U. K. was reporting roughly 2,000 new cases per day. Today, the country is averaging more than 11,000 new cases per day. In the United States, Delta currently causes about a quarter of all new cases and probably will become the dominant strain in our country later this summer.
So, what do we know about Delta?
At a very basic level, Delta is simply a mutated form of the original SARS-CoV-2 virus. The appearance of viral mutants, it should be noted, is not at all unusual. When viruses replicate, they mutate. Their genetic code changes. This process occurs regularly with all viruses. Indeed, one of the reasons that we need to get an influenza vaccine every year is because the influenza virus mutates and changes over time, necessitating a newly designed vaccine. Coronaviruses, like SARS-CoV-2, generally mutate less rapidly than influenza viruses, but mutations do occur. Their genetic make-up does change.
Because mutations occur randomly, these changes typically do not alter the underlying biology of the virus. However, in some cases, random mutations in the genome of a virus can change the ways in which the virus interacts with its host, potentially increasing the virulence of the virus. These mutants are referred to as variants of concern (VOC).
Delta is one of these variants of concern. The mutations present in this variant seem to make it much more transmissible than the original virus. Specifically, mutations in the gene that codes for the viral spike protein enable Delta to bind to and infect our cells more easily. Practically speaking, a person infected with Delta is more likely to spread the virus to other susceptible people. As a result, the number of cases in a population will increase more rapidly. Let’s look at it with a hypothetical example. If a person infected with one virus typically transmits it to two other people, then the number of cases will grow from 1 to 2 to 4 to 8. In contrast, if a person infected with another virus typically transmits it to three other people, then the number of cases will grow from 1 to 3 to 9 to 27. As seen here, an increase in transmissibility will dramatically change the total number of cases in a population.
Can the vaccines prevent infection with Delta?
Because Delta has emerged recently, the effectiveness of the existing vaccines against it has not yet been completely determined. Some preliminary studies suggest that the vaccines may be somewhat less effective at preventing infection with Delta. Other reports suggest that the vaccines remain highly effective. Indeed, in recent reports released by Public Health England, analysts concluded that a single dose of the Pfizer vaccine is somewhat less effective against Delta, but two doses of this vaccine provide equal levels of protection against Delta and Alpha, an earlier VOC. Regardless of the efficacy of the vaccines against variants, we know that breakthrough infections, infections in fully vaccinated people, will occur. Again, that is to be expected. No vaccine is 100% effective. However, if the vaccines are less effective against a particular variant, then more breakthrough infections should be expected.
Is the Delta variant more deadly?
Again, the answer to this important question has not yet been determined. Although the data showing that Delta is more transmissible than other SARS-CoV-2 variants is substantial, information about the severity of disease caused by this variant is less concrete. Some investigators have reported that hospitalizations associated with Delta infections are higher than hospitalizations associated with Alpha (the current dominant strain in the U.S.) infections. However, hospitalizations in the U.K have not increased significantly since the surge in Delta began. Obviously, though, any increases in hospitalizations or deaths will lag behind observed increases in case numbers. Even if the rates of hospitalizations and deaths associated with Delta infections are similar to the rates associated with the other SARS-CoV-2 strains, though, the increased transmissibility of Delta probably will lead to an increase in both of these categories. As we’ve seen earlier in the pandemic, increased hospitalization rates can overburden our healthcare workers.
What should we expect in the near future?
The Delta variant of SARS-CoV-2 is present in the United States and spreading rapidly. Almost all researchers agree that it soon will be the predominant variant in our country, causing the majority of new infections. Most likely, these infections will occur primarily in unvaccinated people. Because the vaccines have not yet been approved for children under the age of 12, we can expect a large number of infections among younger people. Because vaccination rates are worrisomely low among young adults, we can expect a large number of infections among this group of people. Because breakthrough infections will occur, an increased number of infections among vaccinated people may occur. Whether these infections lead to increased hospitalizations and deaths remains to be seen. However, if the number of cases among young people increases, then schools once again may be forced to consider a switch to remote learning.
The good news? The mitigation efforts we have practiced over the past 18 months remain effective. Meeting outdoors, wearing masks, and maintaining a safe physical distance from others can effectively limit transmission, regardless of the transmissibility of this variant. If Delta continues to spread, some of these mitigation efforts may need to be retained in some circumstances. The COVID-19 pandemic has not ended. But we have the tools to limit its impact.
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.