As breakthrough infections rise from the highly contagious Delta variant of the coronavirus, the precise risk to the fully vaccinated may be confusing.

Northwestern Medicine experts explain why the number of breakthrough infections from the Delta variant are going up, who is at risk and why you may need to take extra precautions depending on your age.

Who gets breakthrough infections?  

Dr. Robert Murphy: “The vaccine is about 90% effective against the Delta variant, so one person in 10 who gets vaccinated and is exposed to SARS-CoV-2 will have a breakthrough infection. Anybody is at risk for it, not just the immune-compromised. It’s athletes. It’s totally healthy people. Certainly, the immune-compromised are at a higher risk because they can’t mount a strong enough immunologic response to the vaccine. Older people may also have a weaker response to the vaccine.

“The end game is most people who get breakthrough infections either have very mild symptoms or no symptoms. They rarely end up in the hospital, and they don’t die.

“The big question is how infectious are they? That’s what we are trying to find out.”

Murphy is the site leader at Northwestern along with Drs. Chad Achenbach and Claudia Hawkins in a study with the University of Illinois at Champaign and Johns Hopkins investigating the infectiousness of vaccinated students who got COVID-19 infections. Results are expected in the next one to two months.

Are we seeing more breakthrough infections from the Delta variant?

Carnethon: “We are seeing breakthrough infections and while we know that no vaccine is 100% effective, it feels frightening.  However, infections will happen and the reassuring thing is that most of the breakthrough infections are not causing severe illness and death.  We aren’t sure why the rate of infections — both new and breakthrough — appear to be picking up.

“One concern is the Delta variant is evading the vaccines. Another concern is any virus that causes a high viral load would break through vaccine protection — and there is evidence that the delta variant does cause higher viral loads earlier in the course of infection. We will never get away from these concerns because the virus will continue to mutate itself into new variants so long as it circulates in the population, and the biggest space it has to circulate is among the unvaccinated.” 

Do we need to change our behavior now that the Delta variant is the main source of breakthrough infections?

Dr. Benjamin Singer: “In places with high vaccination rates and low community transmission, you do not need to change your behavior. Of course, vaccinated people may still choose to wear masks if they or someone in their household contact is at particularly high risk.

“But in places with low vaccination rates and high community transmission, I think it makes clear sense to add back masking when you are indoors as an additional layer of protection from a more contagious variant.