Biology Professor Explains Why the Strain Is So Contagious and How to Protect Against It

Prof. Matt Nugent
Prof. Matthew Nugent

By Edwin L. Aguirre

The Delta variant is the most dominant strain of the coronavirus in America today, accounting for most recent COVID-19 hospitalizations and deaths, according to the U.S. Centers for Disease Control and Prevention (CDC). It was first detected in India last December, and had spread rapidly through that continent and then Great Britain before reaching America’s shores in March.

Biology Prof. Matthew Nugent, who is an expert in disease biology, biochemistry and biotechnology, has been studying human diseases, including the coronavirus, at the molecular level for more than 30 years. Nugent, who serves as associate dean for research, innovation and partnerships in the Kennedy College of Sciences, recently answered questions about the Delta variant and how best to protect against it.

Q. How did Delta develop, and what makes it so contagious?

A: The Delta variant evolved through natural selection. Every time the genetic material of the SARS-CoV-2 virus is replicated inside a host cell – that is, a human cell within an infected individual – random mutations can occur that lead to a change in the structure and function of one or more of the viral proteins. When a mutation causes the virus to be more infectious, then that mutant will have an advantage over others and will be replicated more frequently and transferred to other hosts more effectively.

Coronavirus1 Image by National Institute of Allergy and Infectious Diseases, NIH

This colorized, highly magnified view of SARS-CoV-2, the virus responsible for COVID-19, was captured with a transmission electron microscope. The crownlike structures on the surface of SARS-CoV-2 are spike proteins that the virus uses to attach to, penetrate and infect healthy host cells.

The Delta variant has a number of mutations compared to the original virus strain. In particular, it has mutations that alter the structure of a protein, called the “spike,” on the surface of the virus. The spike protein is responsible for binding to the surface of a host cell. Once bound to the host cell, the spike protein changes its shape and burrows into the host cell’s membrane so that the virus can insert its genetic material into the cell, where it can be replicated to produce more viral particles. Delta’s spike protein binds to the host cell more tightly and inserts into the cell better. So, each time a Delta virus bumps into one of our cells, it has a higher probability of sticking and getting in so that it will be replicated.

Delta’s spike protein is also missing certain parts of its structure. That allows it to elude part of our body’s immune system. All of these variations make the Delta strain significantly better at infecting and replicating when compared to the original strain, or even to other prevalent variants such as the Alpha strain.

Q. Are the current vaccines effective against the Delta variant?

A. Absolutely. They are very effective at preventing Delta from making us very sick. The initial infection of the SARS-CoV-2 virus generally occurs in the upper respiratory track. This can still occur in vaccinated individuals – although to a significantly lesser extent than in unvaccinated people – but what is important is that our body’s immune response, which was induced by the vaccination, will kick in and prevent the viral infection from spreading.

Coronavirus2 Image by National Institute of Allergy and Infectious Diseases, NIH
This colorized scanning electron microscope image shows SARS-CoV-2 (orange spheres) – isolated from a patient in the U.S. – emerging from the surface of cells (light green) cultured in the lab.
In particular, vaccination has caused our bodies to store what are called “memory” cells that, once re-activated by viral infection, will secrete antibodies, called IgG, that will prevent the virus from continuing to infect our cells. This is why we are seeing very few cases of severe disease from Delta in vaccinated individuals, and even fewer deaths compared to those who are unvaccinated.

Q. Why are there still breakthrough cases even among fully vaccinated people?

A. Because the Delta variant infects and replicates much better than the original strain, individuals infected with Delta will carry a thousand times more virus particles in their nose, sinus and throat compared to those infected with the original strain. Thus, a person infected with Delta will be shedding many more viral particles every time that person exhales, talks, coughs or sneezes, potentially leading to community spread. So, when a vaccinated individual initially inhales a large quantity of the Delta variant, it can infect cells in the upper respiratory track, where there are few cells that produce IgG antibodies that neutralize the virus. However, as the virus begins to spread in a vaccinated person’s body, it activates the memory cells, causing them to rapidly replicate and secrete more and more antiviral IgG molecules.

This is why a “breakthrough” infection in a vaccinated person generally leads to mild, cold-like symptoms for a couple of days after infection that rapidly resolves.

In a recent report from the state of Indiana, where approximately half the population is vaccinated, nearly 98% of all new COVID-19 cases this year have occurred in unvaccinated individuals. Thus, vaccination remains our best tool in preventing the spread of COVID-19.

Q. Do you think people need a booster shot? How would it help?

A. It appears that this is the way we are heading. I expect that we will all require a booster to reactivate the immune system so that we maintain a sufficient level of memory cells that are ready to respond if we become infected with the virus.

Q. Can vaccinated people who show no symptoms pass on the virus to the unvaccinated?

A. This isn’t clear. What is clear is that vaccinated people can become infected, leading to a brief period where they carry a fairly high viral load in their upper respiratory system such that they can infect others, including the vaccinated and unvaccinated. I suspect that those with breakthrough infections that result in no cold-like symptoms, or are asymptomatic, are likely to be carrying a lower viral load and would be less likely to pass on the virus.

Q. The CDC recommends vaccinating children – from newborns and infants to adolescents – to protect them against hepatitis B, influenza, pneumonia, polio, DTP, MMR and other diseases. Why are the current COVID-19 vaccines not recommended for children younger than 12?

A. The other vaccines that you mentioned had been tested on young children and proven to be safe and effective in this age group, which led to their approval by the U.S. Food and Drug Administration for use in this population. The COVID-19 vaccines currently in use were not initially tested on very young children. The Pfizer and Moderna vaccines have been or are being tested on young children now, and Johnson & Johnson plans to initiate trials on children soon with its vaccine. The data look very good, and the vaccines appear to be safe and effective in children between the ages of 2 to just under 12. The FDA will continue to review these data and will likely be authorizing the vaccines for emergency use potentially as soon as in the mid- to late fall, pending any unforeseen events.

Pfizer vaccine Image by Lisa Ferdinando/DOD
Vaccination remains our best tool in preventing the spread of COVID-19, according to Nugent. Shown here is a vial of the Pfizer-BioNTech COVID-19 vaccine.

Remember: The virus will continue to adapt with each infection. This is why it is paramount that we all get vaccinated. If one chooses to not get vaccinated, they are making themselves a willing host for the virus so that it will have more opportunity to mutate. Eventually, a mutant strain could evolve that would be even more infectious and more deadly than Delta. The virus can only do this if the hosts – that is, us – provide the opportunity.

The Pfizer vaccine was recently granted full approval by the FDA, and the Moderna and J&J vaccines will likely be approved soon as well. Thus, it is hoped that these additional assurances of the safety and efficacy of these vaccines will help expand vaccination across the nation and the world.

Editor’s Note: You can watch Prof. Nugent’s Spring into Science lecture in April on “The Science of COVID-19 and Its Vaccines” here.