Are vaccinated people infected with Delta *really* as contagious as the unvaccinated?

COVID Variants Infection and Spread

A: Probably not. And when you count the reduction in getting infected in the first place, vaccinated people are much less likely to transmit the virus.

💥 TL;DR: Vaccinated people likely *can* transmit the virus, but this risk is NOT the same as the unvaccinated.

Among growing concerns about Delta, recent news that vaccinated people might be as contagious as unvaccinated people was understandably alarming.

While we lacked direct evidence from the clinical trials on vaccines and transmission, for many months the strong belief based on immunology was that vaccinated people were *unlikely* to transmit, because the primed immune system would ramp up quickly to stamp out viral replication.

We also had several real-world studies testing health care workers regularly (thus picking up asymptomatic infections) showing the vaccines were reducing detected infections, but these studies were all pre-Delta.

❓How has Delta changed the game? As always, the science is not fixed. Delta is very new, and we are learning more each day. But so far, the evidence strongly suggests that vaccinated people are still much less likely to transmit the virus, even with Delta.

Delta does appear very adept at getting into cells and replicating quickly. This means that even with the huge head start of a vaccinated immune system, Delta can potentially make more copies of itself before it is neutralized by the immune system reinforcements.

This has made our estimates of transmission from vaccinated people change from *rare* to *possible*. Especially when case levels are high around you, vaccinated people are more likely to encounter the virus and possibly spread it. It’s this new reality that led to updated guidance on vaccinated people wearing masks in public indoor spaces.

The truth is, while we’ve adjusted our opinion that transmission from the vaccinated is more likely than before, we really don’t have a good estimate of the precise likelihood. Epidemiology during a pandemic requires piecing together a lot of clues from imperfect sources, which is where we currently are with assessing the risk of vaccinated people transmitting Delta.

➡️ Let’s review what we know so far. Buckle up, this is a long one!

First, you likely heard of the high-profile study from Provincetown, Massachusetts that spurred the CDC’s updated guidance last week.

The outbreak of 469 cases traced to summer events was notable for taking place in a state with high vaccination rates. Genomic sequencing of a portion of the tests showed the Delta variant made up close to 90% of infections.

➡️ One troubling stat released from this study was that 74% of cases were among the fully vaccinated. As we’ve written about recently, this sounds surprising on the surface but makes perfectly sense in a highly vaccinated population. If 100% of people are vaccinated, 100% of (the small number) of breakthrough infections will be among vaccinated. For a summary of this statistical phenomenon, read our previous post.

➡️ The next alarming data point from the report was about potential transmission.

Nasal swabs from positive cases who were vaccinated had Cycle threshold (Ct) counts that were as high as unvaccinated cases. Ct counts are believed to be *proxies* of viral load. Another study from Wisconsin this week also reported that positive cases among the vaccinated had similar Ct values compared to cases who were unvaccinated. This news was concerning since similar viral loads in vaccinated people *could* imply they are just as likely to transmit.

While this is one interpretation, many scientists weighed in with cautions about inferring too much about what this data can tell us about transmissibility.

The study itself acknowledged that Ct values are “crude” proxies for viral load. Recall that PCR tests look for viral genetic material. This means both active and non-viable (dead) virus can be detected in the nose, which is often the first port of entry and site of replication for SARS-CoV-2. Vaccinated people likely have much more dead virus in their nose because their vaccinated immune systems are on standby and respond quickly. This means they would be less contagious even with the same Ct value as an unvaccinated person. The lungs are also an important replication point and source of infectious aerosols—vaccinated people may stop the replication process more quickly with less virus moving from the nose to the lungs.

➡️ Another important consideration for transmission is the DURATION of viral shedding.

A recent study of patients hospitalized with Delta infections in Singapore found that vaccinated patients had similar *initial* viral loads as proxied by Ct values but much *faster declines* and better clinical outcomes. While hospitalized patients are obviously extreme cases and not representative of most positive cases, the study does demonstrate faster clearance of the virus in this group, suggesting a shorter duration of infectiousness.

This is expected based on how the immune system works—a vaccinated person’s immune system is already on alert and responds rapidly: B cells start cranking out neutralizing antibodies and T cells start killing infected cells. In an unvaccinated person, it takes the immune system much longer to gear up, meaning they have more time to shed virus.

🙋 What about asymptomatic infection? Does all of this mean that vaccinated people can be “silent spreaders” of Delta?

This is certainly what we all fear. If the vaccine doesn’t reduce asymptomatic transmission, our unvaccinated kids and immunosuppressed loved ones remain vulnerable. The good news is that we have several studies suggesting lower *asymptomatic* infection after vaccination when previous variants were circulating. While vaccines (especially one dose) have been shown to be slightly less effective against Delta, it would be very surprising if vaccine protection against asymptomatic infection was suddenly 100% erased.

Luckily this week brought us new data from the UK REACT study, which randomly tests tens of thousands of people in the population every few weeks. This study design has the benefit of picking up positive cases REGARDLESS of whether people have symptoms and sought out testing.

❓What did they find?

💥 Fully vaccinated people had a 50-60% reduced risk of any infection, including asymptomatic infection, compared to unvaccinated people.

💥 Double vaccinated people were less likely than unvaccinated people to test positive after coming into contact with someone who had COVID-19 (3.84% vs 7.23%).

💥 Vaccinated positive cases had LOWER viral load as proxied by Ct values compared to unvaccinated cases.

This study did not break down vaccine type, but it’s important to remember that many in this study would have been vaccinated with Astrazeneca which has shown reduced effectiveness against symptomatic infection with Delta compared to Pfizer (but still strong protection against hospitalization).

Because the REACT study is the only one of the recent Delta studies that uses population sampling and thus catches both asymptomatic and symptomatic cases, we place a higher weight on these findings.

➡️ This study suggests that vaccinated people have a much lower chance of a detectable infection in the first place. If you are not infected, you can’t transmit. When vaccinated people do test positive even asymptomatically, they likely shed less virus for a shorter period of time due to the quicker ramping up of immune responses.


➡️ Vaccines significantly reduce the risk of transmission, though less so for Delta than previous variants.

➡️ It’s hard to put a precise number on the risk of transmitting, so caution is warranted depending on your personal exposures and close contacts who are unvaccinated or immunosuppressed.

➡️ With cases surging, it’s smart and cautious to recommend masking even for vaccinated people—risk reduction is additive. Remember the Swiss Cheese!

➡️ Vaccines are still AMAZING at preventing hospitalization and death from Delta.

Those Nerdy Girls


CDC MMWR report on Provincetown outbreak

Wisconsin study

Singapore study

Great twitter thread from virologist Angela Rasmussen on P’town study

REACT study from the UK

Previous DP post on transmission studies

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