Breakthrough COVID cases: What we know so far

Fully vaccinated people can still contract the novel coronavirus, including the Delta variant, but COVID-19 infection is far more common and widespread among the unvaccinated

A little more than half of all Californians age 12 and older are fully vaccinated, but that’s not enough to prevent COVID-19 from spreading, as evidenced by the recent surge in cases fueled by the highly contagious Delta variant.

While new COVID-19 infections overwhelmingly occur among unvaccinated people, even fully vaccinated individuals can contract the virus. However, such cases are rare and far less likely to cause serious illness, says Shangxin Yang, PhD, a pathologist at UCLA Health.

Here, Dr. Yang explains breakthrough infections, viral load and the best ways to protect against COVID-19 variants.

What is a breakthrough infection?

“Breakthrough” refers to when an individual still gets infected despite vaccination with the full dosage. In other words, they still get infected after they’ve gotten both doses of a two-dose vaccine, or after they’ve gotten the single-dose vaccine.

Is that because the vaccines are not 100% effective?

There are two factors to consider: the vaccine and the host.

The vaccine itself is not 100% effective. There’s a range of effectiveness from 95% to 60%, depending on what vaccine we are talking about. But there’s also the host issue, which is very important.

Individuals who are not immunocompromised can mount antibodies after vaccination very quickly, and they usually generate a substantial antibody level. Immunocompromised patients may or may not have antibodies after vaccination because their immune system may not have responded effectively to the vaccine.

Some immunocompromised patients could very transiently have antibodies, where they develop initially and, after certain immunosuppressive treatments — such as systemic steroid treatments — they disappear. So it's not that straightforward in terms of getting protection from the vaccines. These host issues have an impact on overall efficacy.

What kinds of circumstances might lead to breakthrough infections?

If a patient is immunocompromised, or they receive any immunosuppression or immunosuppressive therapy, they could still be infected even if they’re fully vaccinated. These are the breakthrough infection cases we see most commonly in the hospital.

On a community level, there still could be some healthy individuals who are fully vaccinated and they could still be infected asymptomatically. These people can still potentially be transmitting the virus to people who are susceptible, meaning those who are either not vaccinated or immunocompromised.

Still, the vaccine can reduce asymptomatic infection very dramatically – the reduction is estimated to be about 80% compared to asymptomatic infection in an unvaccinated person.

Is a vaccinated person less likely to transmit the virus than one who is unvaccinated? Does viral load play a role?

Studies show that even though vaccinated people can get infected, their viral loads (the amount of virus present), as well as the duration of the viral shedding (how long viral particles are released during daily activities) are much lower compared to individuals without the vaccine. In that regard, the vaccination itself reduces the transmission rate.

Without vaccination, COVID-19 is transmitted largely by asymptomatic people. It’s estimated that between 50% and 60% of transmission is by those who are infected but without symptoms, so they don’t even know they are infected. Because they are unvaccinated, they might have a very high viral load, and that plays a big role in the course of the illness and likelihood of spreading the disease.

After vaccinations, such instances are dramatically reduced and the overall asymptomatic infection rate is much lower. Even if these individuals do become infected without symptoms, the viral load can be lower and the time period of shedding is narrower, so overall the chance of these people spreading the virus is much lower.

Do specific settings contribute to breakthrough infections?

Vaccination itself doesn’t really stop transmission. It’s more to protect people from getting sick with severe symptoms, being hospitalized or dying from COVID-19. It can effectively reduce transmission, but it doesn’t totally abolish it.

When we do not have herd immunity — meaning when we do not have at least 70% or 80% vaccination rate — we create a scenario where we have a mixture of vaccinated people with unvaccinated people. In such a setting, the virus can still be transmitted between the two groups.

The problem is that unvaccinated people are mostly the ones who get infected, they develop more severe symptoms and they can die. That’s why unvaccinated people are still very vulnerable. They can’t assume that they are protected because other people are vaccinated.

It has to be that the majority of people are vaccinated. Then those who are not able to be vaccinated, such as children under age 12 or people with certain medical conditions, can be protected.

How risky is it for vaccinated people to hang out with unvaccinated people?

In whatever circumstances, it’s just not good for unvaccinated people. The risk is lower than last year, when nobody had the vaccine. But it’s still there, and they can still develop very severe disease.

That’s something they have to understand, that unvaccinated people cannot count on other people to get vaccinated and they will be protected. They, themselves, need to get vaccinated to get really effective protection.

What about vaccinated people spending time with other vaccinated people who might have been exposed to COVID?

It really depends on the host. For healthy people who are fully vaccinated, it is very safe to hang out and mingle together, regardless of exposure. In other words, they can socialize normally. But for immunocompromised people, even with full vaccination, they should still take precautions such as wearing a mask, to protect themselves from getting infected.

Are breakthrough infections more likely in people who were vaccinated some time ago?

So far, all the data have shown that antibodies stimulated by the vaccine can stay there for quite a long time. Because the first available vaccines came out less than a year ago, the data show that after eight months the vaccine is still very effective. I don’t think timing plays a big role there.

What role do the variants, particularly Delta, play in breakthrough infections?

Overall, the vaccines are still very effective against this variant in terms of prevention of severe disease and death. In terms of transmission, though, I do think variants can create a very different dynamic, as demonstrated by the recent surge. That’s because of how different they are from the original viral strain, because all the antibodies and therapies developed a year ago were based on the initial genetic version of the virus.

The Delta variant actually has a lot of differences compared to the original virus, especially in the spike protein. That’s a problem because most of the vaccines were developed based on the original version of the spike gene. This difference has been shown to reduce the efficacy of the vaccines to a certain degree — between 10% and 30%, I’ve seen, depending on the study. Ultimately, the Delta variant is less susceptible to the antibodies mounted by vaccination and it’s highly transmissible.

One study found that people infected with the Delta variant have a much higher viral load. That means that the virus actually can still transmit very efficiently among the general population when we have a mixture of half-vaccinated and half-unvaccinated people. In two months, this variant has taken off and represents more than 80% of coronavirus cases in the country and in California.

The Delta variant really warns us that we need to do more. We need to get more people vaccinated. Otherwise, we’re going to have another variant that’s going to be even more different than the Delta variant, which could reduce vaccine efficacy even further and cause more problems. The virus will not stop its mutation and evolution as long as there’s more transmission and more cases. That’s just the nature of the virus.

What does all this say about the potential need for booster shots?

Many experts strongly believe a booster will be needed to enhance immunity, even if it’s with the same vaccine.

The other question is: If the virus keeps evolving, should there be a new version of the vaccine? That’s the more challenging question, because that would involve much more financial investment and scientific resources. There’s also talk about an annual vaccine, similar to the flu vaccine. That’s all on the table right now.

We don’t know the answer because the situation is so fluid. Six months ago, I was very optimistic. I was hoping that in the summer the virus would pretty much go away, but that didn’t happen.

Further reading

COVID-19 timeline: Track the latest developments

Back to school: It’s almost time to return to the classroom – but is it safe?

Delta variant of COVID-19 is causing concern, specifically for the unvaccinated

What to know about California’s new digital COVID-19 vaccine record


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