The Delta variant caused an inflection point in the battle against the Covid-19 pandemic in the United States. So-called “breakthrough” cases, or Covid-19 infections in people who have already been vaccinated, upended the understanding of whether people in America needed to continue wearing masks to prevent spreading the coronavirus.
But breakthrough cases remain rare, and hospitalization and death for vaccinated people is “effectively zero” in many US states reporting this data.
Here, experts answer key questions to better understand the balance between the newly understood risk of transmission of the Delta variant with the strong protection vaccines continue to provide.
Why are we hearing about breakthrough infections now?
Breakthrough infections were the focus of a CDC investigation that led to a change in mask guidance for vaccinated individuals. The most important caveat here is that they are breakthrough infections of the Delta variant.
The decision to change guidance occurred after the CDC found 74% of the people sickened in an outbreak of mostly Delta infections 469 people in Massachusetts were fully vaccinated.
This came as a shock to the CDC – the understanding prior to the investigation was that fully vaccinated people were unlikely to transmit Covid-19. Delta changed the game, because it is so much more infectious than ancestral strains of Covid-19.
At the same time, a recent report on state-level data from the Kaiser Family Foundation (KFF) found the rate of breakthrough cases in fully vaccinated individuals is “well below 1% in all reporting states, ranging from 0.01% in Connecticut to 0.29% in Alaska”. That means breakthrough cases are very rare.
So, how should we balance these two competing pieces of information? One way, is to think about how we don’t know how many people were protected by the vaccines in Provincetown, a high density summer tourism destination.
“Yes, it’s true 74% of the cases in Provincetown were among vaccinated individuals, but we’re talking about thousands and thousands of vaccinated people coming through Provincetown at that time,” said Dr Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston and Co-Director of the Center for Vaccine Development at Texas Children’s Hospital. “So, I think the vaccines were still performing at a very high level.”
Instead, what appears to be happening is vaccines prevent serious illness and death to a very high degree, but “we may be losing some of that second performance feature,” of decreased transmission.
Research from China helps explain why. In a small study from health authorities in Guangdong province, they found the viral load in the respiratory tract of patients infected with delta was about 1,000 times higher than ancestral strains.
This is important because it goes to how vaccines provoke immunity. All vaccines authorized for distribution in the US are intramuscular injections. They prompt a high degree of immunity, but the body’s ability to recognize and bind to the virus is primarily in antibodies circulating in the blood. There are fewer of these antibodies in the respiratory tract, which has its own special kind of antibody. Theoretically, this could mean Delta’s high viral load overwhelms the immune cells in the respiratory tract, while still giving strong overall protection. But Hotez is skeptical of one-to-one comparisons.
“The myth perception out there is that vaccinated people are transmitting the virus out there in the community as much as unvaccinated people,” said Hotez. “I think it’s still unvaccinated people who are contributing by far the lion’s share of transmission.”
As a reminder, this was a question early in the vaccine drive. At first, scientists were not certain whether people would unwittingly spread Covid-19, even if they were vaccinated. Here’s an explainer on the same phenomenon from May.
How worried should we be about breakthrough infections?
In the same recent report on state-level data, KFF found the rate of hospitalization and death in breakthrough cases was so low as to be “effectively zero” in most reporting states.
Further, it was not known whether Covid-19 was the cause of death in all cases where fully vaccinated and infected people died. In other words, there is a possibility some people died with Covid-19, but not because of Covid-19.
“The vast, vast, vast, vast, vast majority of breakthrough cases end up being no more serious than the flu and typically much less than that, less than a cold and a flu,” said professor Nir Menachemi, chair of the health policy and management department at the Indiana University Richard M Fairbanks School of Public Health, and an expert in hospital surge capacity, emergency preparedness and public health administration.
“If we weren’t in this heightened awareness of the pandemic – and doom is gloom – would we be really that interested in this?” said Menachemi. “Especially when we have unvaccinated people showing up in hospitals on ventilators and morgues?” He doubts it.
Much of what we know about the number of breakthrough cases comes from states. The CDC is not collecting data on infections in people who have been vaccinated, only on people who have been hospitalized or died with breakthrough infections.
Some researchers, like Menachemi, believe tracking all confirmed breakthrough cases would drain valuable resources and create “concern among people where there really doesn’t need to be concern”.
However, not all researchers agree.
University of Washington statistics and sociology professor Adrian Raftery believes there could be a useful measure of Covid-19 cases, but it would be collected more like the unemployment rate, than confirmed total case counts. Rather than simply surveil confirmed cases, he says there should be a national random sample survey of all Americans for Covid-19. Such a measure would help capture asymptomatic infection, because people without symptoms are less likely to be tested, and could be a more accurate window into the prevalence of breakthrough cases.
It’s an idea that builds off of research first done by Menachemi, whose was among the earliest random sample surveys of Covid-19 in April last year, and helped the CDC understand around 44% of all Covid-19 cases are asymptomatic.
“We do random surveys for things we care about – the unemployment rate is estimated by a random survey,” said Raftery. “Breakthrough infections – it’s a very good example of the kind of data that needs to be tracked and should be tracked because the costs of making bad decisions are just huge.
“If we go overboard on public health we hurt the economy. If we don’t do enough on public health we hurt people’s health, and then we hurt the economy as well,” said Raftery. “These are difficult decisions and we need the best data, and we just don’t have it as a nation, and some other countries actually do.”
So what should I do to protect myself and others from Delta?
First and foremost: get vaccinated.
Vaccines are now readily available in most of the US, and in some areas you can even get a vaccine administered in your home. Multiple scientific panels have found the side effects of getting vaccinated, which are generally short-lived and mild, are far outweighed by the potential risk of contracting Covid-19.
If you’re already vaccinated, take note of Covid-19 transmission is in your region, and CDC guidance. If your city, county or state is an area of substantial or high transmission, then you should where a mask in public indoor settings even if you’re vaccinated.
The CDC has also recommended staff and students in schools wear masks, in part because children under 12 are not eligible for vaccinations. Experts such as Hotez believe people who use immunosuppressant medication or who are elderly may be more vulnerable to breakthrough infections, even if they have been vaccinated.
There is very little data on “long Covid” in breakthrough infections. What little data we do have comes largely from a New England Journal of Medicine study by researchers in Israel, who found 19% of people with breakthrough infections had “persistent” symptoms.
But, even then, breakthrough infections are rare, and rarer still are cases of people who become hospitalized. Deaths are so rare that, according to state data, they are “effectively zero”.
“We need to get out of the cycle of focusing on breakthrough infections, which I think just increases the anxiety for vaccinated people,” said Menachemi. “I’m not saying [vaccinated individuals] shouldn’t be vigilant … They can still be infectious.”
“But I think it’s the wrong place that we, as a nation, ought to be focusing on,” said Menachemi. “We don’t end this pandemic until we get the vaccination rates up.”