How vaccines reduce transmission

There is a common misconception that vaccinated people spread the virus as much as unvaccinated. I even received blowback from my holiday post because I said that a room full of vaccinated people with no other precautions didn’t pose a significant threat on the community.

I’m not surprised of the confusion given the suboptimal messaging from public health officials over the past year. But, vaccinated people do not spread the virus as much as unvaccinated. Not even close. Here’s how it works…

Vaccines prevent infection in the first place

Let’s say a vaccinated person and an unvaccinated person are standing next to each other and both are exposed to the same amount of virus for the same amount of time. An equal playing field. The virus then enters both of the people’s nasal passageway. What happens next depends on vaccination status:

  1. For the unvaccinated person, viral particles try to find and invade cells. Once a virus particle enters the cell it starts replicating fast. We need a certain amount of virus to become contagious. The virus reaches this threshold and the person starts transmitting to others. The person is contagious for 24-48 hours before getting disease (i.e. showing symptoms).

  2. For the vaccinated person, the viral particles try to find host cells but the immune system (and particularly neutralizing antibodies) recognize the virus and quickly destroys it. Importantly, the virus is destroyed before entering host cells and, thus, cannot replicate. Because it can’t replicate, the vaccinated doesn’t become contagious. This phenomenon is called “sterilizing immunity”, which prevents infection from happening in the first place. Not everyone gets sterilizing immunity, but COVID19 vaccines help with approximately 50-75% reduction in initial infection risk.

This has a huge effect on transmission in the community. You cannot transmit an infection you never get.

Reducing transmission among breakthrough cases

But, vaccines aren’t perfect. For some unlucky few (and specifically those with high exposure jobs like healthcare), vaccine-induced immunity won’t be able to catch all the virus particles before finding cells. Once the virus finds a cell, it starts replicating enough virus to make the vaccinated person contagious. Then, this person typically gets asymptomatic disease, but some breakthrough cases get mild to severe disease.

The vaccine still kicks in though, making breakthrough cases less contagious than unvaccinated cases. Vaccines do this in two ways:

  1. Clears the virus faster. The vaccinated are contagious for far fewer days than unvaccinated (average 3-6 days vs. 13-18 days with Delta). We’ve had three studies provide this evidence thus far: one in Singapore (here; see figure below), one among NBA (basketball) players (here), and one in the UK published in the Lancet (here). The faster the virus is cleared, the less it’s transmitted.

  2. Reduces number of infectious particles. In the first few days, breakthrough cases have the same number of virus particles as unvaccinated (this is called viral load). But viral particles do not equal infectious particles. In fact, vaccinated have less infectious viral particles than unvaccinated. For example, in a study with healthcare workers, the vaccinated and unvaccinated had the same viral load. However, 69% vaccinated were positive for infectious virus compared to 85% unvaccinated positive for infectious virus. We also saw this in another study in China (here). The less infectious virus particles we have, the less the virus is transmitted.

So, in conclusion, the majority of vaccinated people won’t spread the virus if they are exposed. Among breakthrough cases, vaccines ensure less infectious virus for a shorter period of time. Together, transmission is significantly reduced.

Boosters reduce transmission even more

Unsurprisingly, boosters reduce transmission because they increase our neutralizing antibodies. Boosters increase the likelihood of preventing infection in the first place and, presumably, helps clear the virus faster among breakthrough cases. This week a preprint was released assessing this phenomenon for the first time. The study was a lot of mathematical models, but essentially a booster made a sizable impact in reducing individual transmission. Because of this, boosters deceased community transmission by 21-66%.

What about infection-induced (“natural”) immunity?

Some that are unvaccinated and survive COVID19 disease mount an immune response. This means they too have neutralizing antibodies and, because of this, their body acts much like the vaccinated: prevent infection and, thus, prevent transmission.

The problem is that this response is not guaranteed. For example, people with asymptomatic or mild disease may not mount a strong enough response for variants of concern but people who survive severe infection (i.e. hospitalization) do (here). If they don’t mount a response, they don’t have enough neutralizing antibodies to fend off infection and thus transmission.

If the unvaccinated mounts an immune response, the durability of protection is also variable. Infection-induced immunity lasts for at least 90 days and a maximum of 5 years. Mathematical models found that, on average, people will mount immunity against SARS-CoV-2 for 16 months.

Because of this, reinfection is much higher among unvaccinated. Two peer reviewed studies found the reinfection rate is 2.5-5 times higher among infection-induced immunity compared to vaccine-induced immunity (here, here).

Bottom Line

The majority of vaccinated people won’t spread the virus if they are exposed. Among breakthrough cases, vaccines ensure less infectious viral particles for a shorter period of time. Fully vaccinated people provide little to no threat to community transmission. Boosters also help. And those with infection-induced immunity may or may not help reduce transmission.

We desperately need pandemic off-ramps. What is our plan to transition into an endemic state? I continue to be surprised and disappointed that we still don’t have guidance on this from public health officials. But, a very safe “off-ramp” is vaccinated people. A room full of vaccinated people, for example at Thanksgiving, with no other precautions poses little to no threat to the larger community.

Love, YLE

P.S. I originally included how vaccines prevent long COVID19 too, but this post was getting too much for a Saturday morning. I’ll explain in the next. Stay tuned.

“Your Local Epidemiologist (YLE)” is written by Dr. Katelyn Jetelina, MPH PhD— an epidemiologist, biostatistician, professor, researcher, wife, and mom of two little girls. During the day she has a research lab and teaches graduate-level courses, but at night she writes this newsletter. Her main goal is to “translate” the ever-evolving public health science so that people will be well-equipped to make evidence-based decisions, rather than decisions based in fear. This newsletter is free thanks to the generous support of fellow YLE community members. To support the effort, please subscribe here: