The Delta variant — a newer and more contagious version of the SARS-CoV-2 virus — made headlines this summer when it was found to be responsible for a cluster of COVID-19 cases originating in Provincetown over Fourth of July weekend. Public health officials have confirmed the Delta variant is now widely circulating throughout the U.S.; it’s behind a sharp increase in infections locally and nationally among unvaccinated people and even some vaccinated individuals.  

Hospital Epidemiologist Michael Klompas, MD, MPH, recently spoke with the Brigham Bulletin about what we’ve learned so far about the Delta variant.

Michael Klompas

What’s the biggest concern with the Delta variant?

MK: The key thing about Delta is that it’s much more contagious compared to prior versions of this virus. To quantify that, we look at how many people on average someone with COVID infects.

With the original version of the virus that came out of Wuhan, China, each person infected two and a half people on average. The version that caused Europe’s first wave brought that up to about three people. Then the Alpha variant — which affected the United Kingdom this spring and hit Michigan pretty hard — infected an average of four to five people. Delta is even more contagious, infecting about five to eight people on average after exposure.

What makes Delta so contagious?

MK: Someone infected with Delta has much, much more virus inside their body compared to the prior variants. It’s simply far more successful at reproducing.

When there’s more virus, two things happen. The first is that it’s more likely to lead to severe disease. The second is that it’s more likely to spread.

Every time someone infected with Delta exhales, they’re putting out that much more virus — maybe a thousand times as much compared to Alpha. More virus means that it will take more time and distance for the amount of virus to dissipate to safe levels and more potential for enough virus to get around a poorly fitted mask to cause infection.

How is Delta affecting our inpatient census at the Brigham?

MK: We’re seeing an increase, but we’re nowhere near the census we had back in March and April, nor what we experienced over the winter. It is also much, much lower compared to what we saw during the initial spring surge in 2020.

However, cases are still rising, and I think we are still in the early upward-swing phase of this Delta outbreak. So, we do expect our inpatient census to continue to gradually rise.

Does vaccination protect against Delta?

MK: Yes. We know that the mRNA vaccines, Pfizer and Moderna, are active against Delta. They remain very effective overall, particularly against severe disease, but they’re a little less effective against mild disease and less effective compared to their protection against prior variants. Whereas we had 95 percent protection against Alpha, with Delta it comes down to about an 88 percent level of protection.

What we are learning about Delta is its capacity to generate very high viral loads gives it the opportunity to overwhelm the vaccine and cause infection, which can lead to a vaccinated person feeling pretty lousy for a couple of days. But it’s very unusual for a vaccinated person to require hospitalization or die from infection. This variant basically seeks out the unvaccinated population, which is where it is so successful at spreading.

Fortunately, we’re the second-most vaccinated state in the country, and I think this is why the rise of cases overall, and especially hospitalizations, in Massachusetts is not as severe as it is in other parts of the country. Still, one-third of our state’s population — and one-quarter of adults — are not yet fully vaccinated and thus especially susceptible to getting infected.

What did we learn from the Provincetown cluster?

MK: That outbreak is tied to at least a thousand cases so far, nearly all caused by Delta. The Massachusetts Department of Public Health published a nice analysis on about 450 of these cases, and what we learned is that 75 percent involved fully vaccinated people. Of that population, 80 percent developed symptoms. Four of those fully vaccinated people were hospitalized. Thankfully, nobody has died at this point.

In addition, we’ve learned that the viral burden in those vaccinated people is the same as it is in unvaccinated people, and it’s high — meaning that these individuals can transmit the virus to other people.

Are there differences in how Delta affects vaccinated and unvaccinated people?

MK: Yes. The first difference is their period of contagiousness. For the first few days of infection, the amount of virus in a vaccinated person and an unvaccinated person is high and similar. But after that time, the vaccine really kicks in and brings that viral load down rapidly and aggressively. What that means is that that while you are highly contagious during an initial phase of your illness, a vaccinated person is not going to be contagious for as long as an unvaccinated person.

The second difference we see is with symptoms. A vaccinated person will get symptoms around 70 percent of the time whereas that figure is 90 percent for an unvaccinated person. Vaccinated people can get a fever, but about half as often compared to an unvaccinated person. They might get a cough, but about one-third as often and a little bit less for a sore throat. Interestingly, a runny nose may be more common in the vaccinated population.

What’s very important is that the serious symptoms — shortness of breath, hospitalization and death — are highly unusual in a vaccinated person.

Mass General Brigham recently updated its mask guidance for employees. Can you please summarize the changes?

MK: Yes, we no longer permit fully vaccinated employees who are less than 6 feet apart in non-clinical areas to remove their masks. All employees, regardless of vaccination status, must wear a mask (a) at all times in all clinical and public areas of the hospital, and (b) any time they’re within 6 feet of another person, even in non-clinical areas, including conference rooms, workrooms and breakrooms.

It’s a concerning time. There’s a lot of COVID right now in the U.S., and there’s a lot here in Massachusetts as well. Vaccinated people, unfortunately, are still susceptible. If you’re fully vaccinated, you’re very unlikely to get sick enough to be hospitalized or die, but you might have a bad flu-like illness for a couple of days, and you can transmit it to other people. Please be careful. Wear a well-fitting mask. Distance yourself. Make sure you have good ventilation in indoor spaces and avoid dense crowds.

20 Responses to “Q&A with Michael Klompas, MD, MPH: What You Need to Know About the Delta Variant”

    • Yevgenia

      Very helpful! Thank you!
      Q: I was vaccinated as everyone else in First week of February in housing 55+ First dose and later with second dose Pfizer vaccine. Do you still think we are really have immunity some kind left?
      Why so hard to find answers to this question?
      Thank you!

      • BWH Strategic Communication

        Thank you for your question, Yevgenia. Per Dr. Klompas: “The data we’ve seen suggest that immunity gradually decreases over time. Data from Pfizer suggest that the effectiveness of their vaccine decreases from about 95% in the first 2 months following one’s second dose to about 80% at 6 months for infections overall. However, protection against severe illness requiring hospitalization and death is better preserved and remains above 90%.”

  1. Anne Papajiannis

    Do we know how many new cases are Covid-19 versus the Delta variant or are all new cases from the Delta variant?

    • BWH Strategic Communication

      Thanks for your question, Anna. Per Dr. Klompas: “It’s believed that the vast majority of new cases of COVID-19 in the U.S. are now due to the Delta variant.”

  2. Shelly

    Its nice that the Pfizer and Moderna are active against the Delta Variant but the Johnson and Johnson vaccine is not mentioned. So for those like myself that got the J&J vaccine here in hospital as an employee here, am I not protected against Delta variant. J&J vaccine was so quick to be approved by BWH but is it really even effective compared to the other vaccines?

    • BWH Strategic Communication

      Thanks for your question, Shelly. Per Dr. Klompas: “Like the mRNA vaccines, we know that the Johnson & Johnson vaccine is effective against preventing hospitalization and death. At this point, however, there isn’t as much data on how J&J fares against preventing Delta infections more broadly, largely because the mRNA vaccines are more prevalent in the U.S. population and therefore more studied. There are some unpublished data emerging from South Africa where Delta is circulating suggesting that J&J is about 70% effective against hospitalizations and 95% effective against death.”

  3. Laura McCarthy

    Thank you for this information. It’s helpful to be in the know.

  4. Jennifer Bailey

    Thank you for taking the time to educate us. Do we know what the rate of long-term complications is among vaccinated people who got Delta? is it as high as unvaccinated people?

    • BWH Strategic Communication

      Thanks for your question, Jennifer. Per Dr. Klompas: “This is something we are all eager to discover. At this time, we simply don’t know what the rate of “long COVID” symptoms are or will be among vaccinated people simply because infections among vaccinated people are a relatively new phenomenon.”

  5. Martha Rogers

    Thank you for this very helpful resource. What is the percentage rate of likelihood of spreading Delta Covid from one vaccinated person to another when indoors masked and when indoors unmasked? When outdoors masked and outdoors unmasked?

    • BWH Strategic Communication

      Thanks for your question, Martha. Per Dr. Klompas: “The risk of transmission exists along a continuum, with factors that both increase and decrease transmission risk. The factors that increase the likelihood of transmission are high community incidence rates, proximity, sustained exposure, poor ventilation, lack of vaccination, and lack of masking. The converse factors decrease transmission risk: brevity, distance, good ventilation, vaccination, mutual masking, and quality of mask fit & filtration. No one factor is determinative, and it’s difficult to assign a percentage transmission risk for any given encounter without taking all these factors into account. So, the risk of transmission for a vaccinated person to another vaccinated person is lower than the risk of spread with unvaccinated people, and masking will further diminish that. But if you happen to be right next to a vaccinated person with acute infection in a poorly ventilated space for a long period of time, you’ll be at risk for infection — even if both of you are masked. Risk of transmission outdoors is lower, but again, if you’re face-to-face with someone for a sustained period, transmission can certainly occur.”

  6. Donna Lawson

    Any advice to young persons who are still reluctant to receive the covid vaccine?

    • BWH Strategic Communication

      Thanks for your question, Donna. Per Dr. Klompas: “Yes! This virus, and especially this Delta variant, is highly contagious and even younger adults can become very ill. Whereas the first wave of the virus had a disproportionate impact on older adults, we’re seeing now with Delta that unvaccinated people in their 20s and 30s can and do become quite sick. The vaccines are safe, effective and now widely accessible. Even if you get sick after being vaccinated, you are likely to experience far, far milder symptoms than if you were not.”

  7. Catherine Shankweiler

    Considering the Delta variant, when is the ideal time to get tested after possible exposure for a vaccinated person? (for example, 2 days after possible exposure or 3 days…?). I assume if it’s too soon, we will see false negatives?

    • BWH Strategic Communication

      Thanks for your question, Catherine. Per Dr. Klompas: “Testing guidance has not changed due to the Delta variant. You should get tested three to five days after exposure to someone with suspected or confirmed COVID-19.”

  8. Melissa

    Thank you very much for this interesting and helpful information. I have a question regarding masks and the Delta variant. As I have children returning to school in the Fall that are to young for the vaccine at this time. Do you feel that cloth masks (the good double layer ones not the bandana style) are as effective in decreasing the risk of contracting? Or would you recommend medical grade?

    • BWH Strategic Communication

      Thanks for your question, Melissa. Per Dr. Klompas: “There is wide variation between masks in the amount of protection that they offer. Assuming one is using a good-quality, multilayer cloth mask, the most important factor is quality of fit. A poorly fitting mask will allow some respiratory particles (and thus virus) to escape and / or enter around the sides, top, and bottom of the mask. Protection from masks therefore exists along a continuum – the better the fit, the more protection one will get. As such, a good-quality, multilayer cloth mask that fits well may offer more protection than a loosely fitted medical-grade mask.”


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