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Covid-19's delta variant is spreading in America. Here's what to know.

The vaccines in the U.S. are holding strong against delta — for now.

Of the four significant variants of Covid-19 that we've identified so far, the delta variant is the one that should worry Americans — but not for the reasons you would expect.

The mere existence of the variant, along with other related offshoots, such as delta plus, suggest that each set of significant mutations comes with increased transmissibility and possibly resistance to immunity from vaccination. Today's concerns about the delta variant were yesterday's worries about the alpha variant, and tomorrow they could be replaced by fear of the epsilon or zeta variants. Media coverage of the delta variant's spread has been rife with contradictory messaging from public health officials, adding to the confusion.

So what do Americans really need to consider with the Fourth of July past us and summer truly underway? Let's start with what the delta variant actually is: The delta variant (also known as B.1.617.2, first identified in India in October) has rapidly spread around the world, becoming both a variant of concern and the dominant strain for new Covid-19 cases in the U.S.

Case reports from the U.K. indicate that some people can be infected in as little as five minutes of exposure to a carrier.

In the United Kingdom, cases of the delta variant doubled over two weeks in June, a strong signal that this is a more transmissible strain. Case reports from the U.K. indicate that some people can be infected in as little as five minutes of exposure to a carrier. This translates to larger numbers of people who can be infected overall — if, for example, the original strain infected 10,000 people, this strain could infect 40,000 or more in the same population.

Our only defense mechanisms against this and future variants are just that — defensive — and in rank order, they are vaccines, vaccines and vaccines. Recent studies indicate that the Pfizer-BioNTech-manufactured vaccine is effective against the variant but less so than in its protection against the alpha variant that was first identified in the U.K. These data suggest that all three of the vaccines available in the U.S. are likely to confer some immunity, but less than indicated in clinical trials.

So far, we do not believe that the delta variant results in a greater proportion of hospitalizations or deaths — but those data are still being collected and evaluated. Based on what we know so far, here are five practical implications:

  1. If you are fully immunized, you are highly protected from severe illness and death — but you can still get infected with SARS-CoV-2. This was true before the rise of the delta strain, but it is worth re-emphasizing that you can still be infected after a full round of vaccines — over 4,100 breakthrough cases have resulted in hospitalizations or death. This is an incredibly low number compared to what we've seen over the last year and a half, but it is enough to remind everyone that the value of a vaccine is to prevent death and severe disease. We are not sure to what degree vaccination can prevent transmission of the delta variant to unvaccinated people. In an unknown pandemic with a novel virus and even more novel variants, it is important to keep in mind that the goal of immunization has been to keep people alive — and vaccines are the most effective way to do that.
  2. Not all immunity is created equal. We know that there are populations for which immunity by vaccination might not be as durable — including patients who have had transplants, elderly patients and patients with chronic conditions. With that in mind, people should consider their appetite for individual risk in various situations. Being in groups with others who are vaccinated is safe without masks. Being with someone of unknown vaccination status puts the unvaccinated at most risk, but what if you are vaccinated and in a higher-risk category and want to maximize your own safety? It is best to avoid crowded indoor spaces and to choose settings where people will wear masks.
  3. Not all vaccines are created equal. The largest dataset about effectiveness against the delta variant is from a preprint study of over 14,000 infected people that shows 96 percent efficacy of the Pfizer vaccine. Because the Moderna vaccine is almost identical, it should confer similar percentages. Recent data from Johnson & Johnson support that its single-dose shot is also effective, although potentially at a lower rate than the mRNA vaccines. The data were derived from eight people's plasma in a lab-based study, which is not the same type of study that documented the efficacy of the Pfizer vaccine. (In addition, confusing preliminary data from Israel indicate that the efficacy of the Pfizer vaccine against the delta variant might be lower than reported but still sufficient to protect against severe illness.)
  4. We do not know what delta means for children yet. The most common question is what to do for children who cannot be vaccinated or for whom vaccination is not appropriate. The truth is that risk of severe disease and death from Covid-19 is lower for minors compared to adults — but if it happens to your child, those statistics do not matter. Data from the U.K. illustrate that school-based outbreaks in clusters from the delta variant pose risks for unvaccinated children and even vaccinated adults. Bottom line: Children are not little adults. They should be treated with the respect they deserve, and that includes protecting them from an infection that could have short- and long-term implications. The American Academy of Pediatrics recommends that children over age 2 who are not vaccinated should wear masks indoors under most conditions. Outdoor transmission is rare, and with the rolling heat waves, masks outdoors are not necessary or sustainable — but whether it is for travel or camps or schools, consider whether masks are warranted. All facilities that work with children should also screen for active symptoms among all attendees and encourage other forms of disease surveillance, such as testing.
  5. When all else fails, use common sense and basic mitigation measures — we know they work. The Covid-19 vaccines are miracles, no doubt, but if a situation is concerning, adding basic mitigation measures does work, as well. High-quality masks and distance can be controlled at the individual level in almost any circumstance. Adding rapid tests that are available over the counter at most pharmacies can augment mitigation efforts.

The delta variant is here, and it is likely to result in an increase of new cases throughout the U.S., the majority of which will be among unvaccinated people. But the more concerning threat is what an active reservoir of humans in which the virus can replicate and slowly mutate means for the world. Until global vaccination is reached, more mutations will lead to variants that could lead to chances to escape immunity. The rise and spread of the delta variant show us that it is about more than individual choices about whether to get vaccinated — it is about people's role in protecting the world.