Breakthrough infections and the COVID-19 delta variant: What you need to know

Reports of breakthrough COVID-19 infections are on the rise, as the delta variant rapidly spreads in communities across the country.

But what is a COVID-19 breakthrough infection, exactly, and why are certain people at greater risk of them? How much protection do vaccines provide against breakout infections? And what can you do to reduce your chances of developing a breakthrough infection if you’re fully vaccinated against COVID-19, whether you have had the Moderna, Johnson & Johnson or Pfizer-BioNTech vaccine, now called Comirnaty?

Here’s what MD Anderson’s Chief Infection Control Officer Roy Chemaly, M.D., has to say.

What defines a breakthrough infection?

A breakthrough infection is one in which a person who has been fully vaccinated against a particular pathogen still becomes infected with it after being exposed. In the case of COVID-19, it means a person becomes infected at some point more than two weeks after receiving the second dose of either the Pfizer or Moderna mRNA COVID-19 vaccines or the single-dose Johnson & Johnson vaccine.

How common are COVID-19 breakthrough infections?

Despite the increased transmissibility of the Delta variant and what you might hear in the news, breakthrough infections are not actually that common. Most COVID-19 infections still occur among unvaccinated people. That’s why we’re strongly encouraging everyone who can be to get vaccinated.

Nothing is 100% effective. But the Pfizer, Moderna and Johnson & Johnson COVID-19 vaccines all work very well. They are about 80% to 85% effective against the delta variant, which is a little lower than the 95% effectiveness they had against the original strain of SARS-CoV-2. So, we’re not surprised to see some people developing breakthrough infections.

The great news is that most of the individuals who have had breakthrough COVID-19 infections are not experiencing severe illness. Most are not being hospitalized. And most do not die. That is fantastic. Because among the unvaccinated, infections tend to be much more serious, and result in many more hospitalizations and have a much higher death rate.

Are people with breakthrough infections more or less likely to spread COVID-19 than unvaccinated people?

Probably less, but they still could spread the coronavirus. Vaccinated people who show no symptoms are the least likely to spread it, because droplet particles are generated mostly by coughing and sneezing.

Unvaccinated people, meanwhile, are more likely to have a higher viral load. This means that greater levels of virus are circulating in their systems. So, they will release and potentially spread more virus every time they cough or sneeze.

Are COVID-19 breakthrough infections more common with one type of vaccine vs. the others?

Preliminary evidence suggests that the Moderna vaccine might confer slightly more protection than the Pfizer vaccine. But when it comes to protection from serious infection and death, though, both the Pfizer and Moderna mRNA vaccines provide about the same amount.

There’s not much data available on the Johnson & Johnson vaccine yet because it’s newer. So, it’s impossible to compare that one to the others.

Are certain people at greater risk of a breakthrough infection?

Yes. The Centers for Disease Control and Prevention (CDC) has looked at the data and determined that immunocompromised individuals, including some cancer patients, are still at risk, even after receiving the recommended two-dose regimen of the Pfizer or Moderna vaccines. That’s because their bodies are often not mounting a sufficient immune response to the coronavirus after vaccination. This may be due to cancer treatment or other underlying conditions.

However, certain immunocompromised individuals, including some cancer patients and survivors, can receive a third dose of COVID-19 vaccine to help strengthen their immune response. If you meet the criteria, it’s very important to get that third dose as soon as possible.

Eligible immunocompromised individuals can receive an additional dose at least 28 days after your second dose of the Pfizer or Moderna COVID-19 vaccine. But if you are in active cancer treatment, talk to your doctor about timing your third dose with your treatment schedule.

What are the symptoms of a breakthrough infection – and how do they compare to the symptoms people may experience if they get COVID-19 but aren’t vaccinated?

It’s more a matter of degree than kind. Both vaccinated and unvaccinated individuals can still develop symptoms like a cough, fever or a runny nose. They’re just more likely to be severe in someone who is unvaccinated.

Once booster shots become available, why will these be important for certain individuals?

Some data are showing that after a few months, people may have waning immunity against COVID-19. So the FDA and the CDC are meeting now to decide when a booster shot should be given.

A booster is defined in part by timing. Booster shots call for people to get them a specific amount of time (e.g., 6 months or 8 months) after the original dose or series to revive or boost the immune response. That’s why we don’t consider the third dose I mentioned earlier for the immunocompromised a booster. In the immunocompromised, the additional dose is considered part of the initial vaccine series and should be administered any time after 28 days have elapsed from the second dose.

But both third doses and boosters are designed to protect people from serious illness and death, and both may reduce the rate of breakthrough infections.

What precautions can people take to reduce their risk of breakthrough COVID-19 infections?

We’re in the middle of another surge, and it is very serious. The Delta variant is highly transmissible and spreading like wildfire. You can see it in the number of daily infections and the number of hospitalizations reported. It accounts for roughly 85% of all COVID-19 infections right now.

So, we need to roll back all the precautions we’ve started to relax on. And it’s the same party line for everyone – not just the immunocompromised or the unvaccinated.

In order to protect ourselves and each other, we have to practice masking and social distancing again, and avoid large gatherings. And those who aren’t vaccinated yet should not put off vaccination any longer.

Some people see breakthrough infections as proof that the vaccines don’t work or as a reason not to get vaccinated. What do you want them to know?

The main goal of vaccination is not just to prevent infections. It’s to prevent serious illness, hospitalization and death. And all three vaccines are very, very successful at doing those three things and saving lives.

This latest surge is still being driven primarily by the unvaccinated. They’re the ones who are being hospitalized and getting very sick and not doing well.

Most breakthrough infections are much milder than the COVID-19 cases in unvaccinated individuals. So, if I get COVID-19, I want to make sure I’m vaccinated. Because even partial protection may prevent serious illness.

Schedule a vaccine appointment at MD Anderson. Eligible MD Anderson patients can sign into MyChart to schedule an appointment for a third COVID-19 vaccine dose. Appointments for community members who are eligible for a third dose will be available at MD Anderson soon.