The Effect of the Delta Variant on the COVID-19 Pandemic

People working in an library wearing masks during COVID-19

July 29, 2021

Delta Variant

The effects of the delta variant on everything from office reopening plans to the requirement for wearing masks in public locations have changed both the public and governmental attitudes from a sense of being nearly over this, to fears that we are entering yet another major wave. On Wednesday, for the first time since February of this year, the number of new cases exceeded 100,000. However, since roughly half of the population in the United States, Europe, and the United Kingdom are vaccinated and, thus, have much less susceptibility to infection, this actually represents an intensity amongst the unvaccinated population much like the worst days of the epidemic over the late fall and early winter of this year.

But it is different now. As Dr. Wollensky, director of the United States Centers for Disease Control and Prevention, said last week, this has primarily evolved into a pandemic of the unvaccinated. Unfortunately, that is not completely true, as the delta variant has proven itself to be more infectious even amongst vaccinated people, but the spirit of that statement is true in that the real risks for bad outcomes remain almost solely with unvaccinated individuals. With that in mind, there are five key points to keep in mind:

  1. People who are not vaccinated are much more vulnerable than before the delta variant. Even vaccinated people have a non-trivial degree of vulnerability to infection, but still only rare bad outcomes.
    • Delta is 2 to 4 times more infectious.
    • Time/distance guidance is out the window. There is very good evidence that Delta acts more like a classical aerosol and can cause infection with much less exposure. This is why everyone in higher spread areas, which will likely include most of the U.S. population over the next month, is being asked to mask up again.
  2. People who are vaccinated are very well protected against bad outcomes.
    • Vaccinated people can get infected, but unless someone is older (generally over 75) or has other known COVID risk factors, symptoms are usually, but not always, so mild that many don’t realize they have it. In fact, we have had several of our WorldClinic members who thought they had allergies or a cold, get a test just to be careful, and were surprised to find they were COVID positive.
  3. Because older people are vaccinated and thus significantly protected, the proportion of seriously ill patients has skewed much younger.
    • This does not mean that younger people are getting very sick at a higher rate per infection – rather, it means that a much higher proportion of those infected are in younger age groups and this larger number of infected younger people is driving up the hospitalization number.
    • In many parts of the United States, the United Kingdom, and Europe, the elderly populations are vaccinated >90% so their serious disease rate has come way down and their previously high contribution to hospitalizations and deaths has been mitigated.
    • In many areas, because older and otherwise at-risk people have been vaccinated and the Delta variant is so much more easily infecting younger unvaccinated populations, half of all hospitalizations are in younger unvaccinated people. Fortunately, they are not dying at the rate that older hospitalized people did, but it is still impacting health system capacity.
  4. The biggest “risk” to business operations and individual activities is government reaction.
    • To governments, an infection is an infection; governments have been so conditioned to react to increasing case rates, even if the outcomes are rarely bad and the impact on resources is minimized, the government instinct is to do something.
    • But, in fairness to the government, as the number of younger unvaccinated people infected is increasing, the volume of infections and the infectivity of this virus is causing a spillover to vaccinated people as breakthrough infections. Generally, these vaccinated patients are not getting sick and dying, but we are just now learning that they are contributing to the spread, although at a lower rate than unvaccinated people. This last point is very significant:
  5. Unlike with previous versions of the virus, we are learning just over the past week that Delta means vaccinated people can contribute to the spread, but at a much lower rate than unvaccinated.
    • Vaccination is highly protective against bad outcomes, but people still need to be careful to reduce their risk of being part of a chain of transmission.


That last point alters some of the guidance we have given in the past weeks. It now appears that people who are vaccinated can carry the virus to other people although at a much lower rate. Right now, this is one of the most important areas of research: What is the relative susceptibility to infection for vaccinated people and, if infected, what is the risk of these people transmitting to others, especially others at home who may be unvaccinated, such as children, elderly, or those who are immunocompromised.

In Dr. Walensky’s brief on Tuesday, she announced the new CDC recommendation that all people, vaccinated and unvaccinated, should wear masks in public indoor spaces whenever the case rate exceeds 50 cases per 100,000 per week (a little over 7 cases per 100,000 per day), she referred to new data that indicates that with Delta even vaccinated people can carry high viral loads in their nose and throat. Since these nose and throat viral particles are what transmit the infection, this implies that vaccinated people can be effective transmitters. The CDC has not released the details of these studies and there is some evidence that these may be based on international studies with vaccines not used in the United States, but for now, we have to take the CDC at their word, although this is very different from experience with previous variants and from studies done in the United Kingdom, which showed just the opposite. Because we have not seen the data, we do not know if this finding applies mainly to symptomatic infections only, in which case, the primary message to vaccinated people would be to double your attention to any upper respiratory symptoms and test early, or if it applies to asymptomatic vaccinated people also.

Quoting the CDC: “A growing body of evidence indicates that people fully vaccinated with an mRNA vaccine (Pfizer-BioNTech or Moderna) are less likely than unvaccinated persons to acquire SARS-CoV-2 or to transmit it to others. However, the risk for SARS-CoV-2 breakthrough infection in fully vaccinated people cannot be completely eliminated as long as there is continued community transmission of the virus.”

This announcement and the assertion of increased risk of Delta-related cases breaking through vaccine protection is what has driven up people’s concerns. Most importantly, while many understand that their vaccination protects them from bad outcomes, they are worried that if they are infected at work, they can bring it home to their children or at-risk family member and then have a worse outcome. While it’s true that is possible, if Delta has increased the risk for people working in a location with very high vaccination rates or mandatory vaccination, multiple low-risk events would have to happen:

  1. A vaccinated person would have to get infected outside the workplace. This chance depends on the incidence in the community, but even at a background level of 25 cases per 100,000 per day, almost 4 times the new CDC threshold for masking in public places, with an estimated efficacy against infection at 80% for the mRNA vaccines, this means that just this first step has been reduced to a risk of about 5 cases per 100,000 per day.
  2. That person would need to develop their infection to the point that they are effectively spewing virus in the workplace, which likely means they need to be somewhat ill. This risk is harder to estimate, but with Delta, people are developing symptoms much more readily than with prior variants and the incubation period is shorter, so if workers know they need to continue to monitor for even mild symptoms and take themselves out of the workplace and get tested, the risk of an unrecognized infection is greatly reduced.
  3. Another vaccinated person would need to catch the virus from the first vaccinated person, and unless the CDC has more data that they have not shared, there are multiple studies showing that the risk of passing from vaccinated to vaccinated is significantly reduced.
  4. This next person would need to develop an asymptomatic infection or ignore symptoms and transmit it to their child, and as with earlier, this asymptomatic state appears to be reduced with Delta.
  5. The child would need to not only need to become infected but be one of the few children that develop severe enough disease to be hospitalized. In fact, the hospitalization rate in children over the entire course of the pandemic has been about 8 cases per 100,000 and these hospitalizations were dominated by children with obesity, chronic lung disease, and for those under 2, prematurity. Together, these and other co-morbid conditions accounted for over 40% of all pediatric hospitalizations, meaning that the hospitalization rate for children without underlying medical conditions is about 5 per 100,000 with the death rate about 1/500th of that and almost completely confined to the higher risk group. Compare both the overall 8 per 100,000 and the 5 per 100,000 over the 18 months of the epidemic to the pediatric hospitalization rate for influenza which is about 11 per 100,000 in a typical 6-month flu season.

So, while there are concerns about bringing the virus home to children, keep these risks in perspective and recognize that many unlikely events would have to occur for this to happen. And, the incidence of problems that are scary, but may not require hospitalization, such as myocarditis, or Multi-system Inflammatory Syndrome are not to be ignored but are low and typically self-limited, but high enough that we do want to make sure we are protecting children by vaccinating them when available for their age and paying attention to our own symptoms.

Booster Shots

There has been much discussion over the last week about the need for boosters considering the delta variant. Pfizer has announced that they are submitting emergency use authorization for a third dose. Additionally, the Advisory Committee on Immunization Practices at the United States CDC last week presented data that an additional shot as part of a primary vaccination series should be considered for those over 75, people taking immunocompromising medications, and those with blood but not solid organ cancers. This was not a formal proposal at this point, but a preliminary presentation that was favorably received. This same group has reviewed data on a true booster and says that based on the data to date, while there may be drop-offs in the simple antibody measurements, this drop-off appears to be more than compensated for by the maturation of cellular immunity components. At this point, it still appears that for most people there would be little benefit to a booster, although the door is still open on a modified vaccine to better cover specific variants, such as delta.

Delta Timeline

Lastly, we have some projections on the course of the delta-driven wave. If we look at the course of the delta epidemics within the pandemic, we see that in both India and the United Kingdom, the delta wave went from unimportant to a high peak in 7 to 8 weeks, with a similar decrease from the crest. Since Delta is so infectious, it tears through unvaccinated populations, eventually using up the available hosts until the combination of previously infected and vaccinated starts to apply herd immunity effects to the wave. If we apply that to the United States, our Delta wave started around July 1st, so it is a reasonable guess that we will follow these international experiences and our delta wave should peak around the end of August. A concern will be whether or not schools returning right around that time will sustain the wave, but with the CDC guidance to have kids mask up, with continued efforts at social distancing in schools, and with the pre-delta experience that showed schools were not effective petri dishes, which one does have to look at with some degree of skepticism, there is a good chance that schools will not be the location that sustains the outbreak.


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