There have been a few scary-sounding headlines lately when it comes to getting sick with Covid-19 after being vaccinated:
The thing is, none of these headlines should be surprising, or all that scary.
According to clinical trial data, the Pfizer/BioNtech vaccine is 95 percent effective at preventing Covid-19 disease. The Moderna vaccine is 94 percent. The one-dose vaccine from Johnson & Johnson is between 66 and 72 percent effective (and higher at preventing severe disease). What you’ll notice in all those figures: None is 100 percent effective at preventing the illness.
This should be obvious from the clinical trial data, but it’s catching some by surprise: It is possible to get sick with Covid-19 — or possibly carry the virus asymptomatically — after being fully vaccinated with any of these vaccines. These are called breakthrough infections. “This is what we expect to see: some level of cases among vaccinated people,” Natalie Dean, a biostatistician at the University of Florida, says.
It’s also not surprising to see a few (extremely rare) cases of hospitalizations and severe disease among people who have been vaccinated. That may seem even more puzzling since the clinical trials of tens of thousands of people reported that these vaccines were 100 percent effective at preventing hospitalizations.
But “nothing is 100 percent,” Dean says. “When you start to talk about vaccinating millions of people, even things that occur relatively infrequently will start to pop up.” The real culprit behind the breakthrough infections is not the vaccines — it’s the fact that this pandemic is still raging in many communities.
But why would a vaccinated person ever get sick?
So far, I’ve been talking about vaccine efficacy in terms of preventing disease. There’s also the question of the vaccines preventing infection. It’s slightly different: Disease is showing symptoms; infection is simply testing positive for the virus (perhaps asymptomatically). On this, so far, it appears the vaccines are really good at preventing infection as well. A recent study from the Centers for Disease Control and Prevention of 3,950 health care workers found that the mRNA vaccines (Moderna and Pfizer/BioNtech) were 90 percent effective at preventing any infection.
There’s less real-world data on the Johnson & Johnson vaccine, but the CDC writes that early evidence suggests it “might provide protection against asymptomatic infection” as well.
Which is all to say: Most of the time, these vaccines will prevent Covid-19 infection from taking hold in the body. That’s great! And it’s why these vaccines are our best shot at ending the pandemic. But some of the time, it won’t prevent infection, and won’t prevent symptoms.
Why? Think of the immunity that is conferred by the vaccine as a dam, says Erin Bromage, an immunologist at UMass Dartmouth.
“No dam, for example, is 100 percent effective,” he says. “They’re designed to handle 20-years storms, 50-years storms. But they get overwhelmed when a 100-year storm comes along.”
But there are circumstances in which it can overwhelm this immunological dam.
The exact reasons aren’t precisely known. Some of the circumstances leading to a breakthrough “can be predicted, but others, they just can’t be,” Bromage says. “You don’t know who it will be.”
Just like people have varying immune reactions, and varying disease severity when they catch the virus, people have varying (but less so) immune reactions to vaccines. (It’s also like how some people get a lot of side effects from the vaccine, and others do not. Everyone’s body just responds a little differently.)
So, based on what we know so far, a big part of who will experience breakthrough infections is just hard to predict.
But there are also some things that scientists suspect might make it more likely.
One might be the quantity of the virus a person is exposed to. Like how a dam will be breached when a huge flood comes rushing in, the immune protection from vaccines could be overwhelmed when a person is just exposed to a lot of viral particles (perhaps by living with someone sick with Covid-19, or by working in a health care facility treating Covid-19 patents).
Other times, “the foundation that the dam was built on is not strong enough,” Bromage says, leading the dam to break. A person who is immunocompromised might be more at risk for breakthrough infection. “We know, for example, that people who have HIV had lower levels of protection,” Dean says. “It may reflect something about the immune response.”
Age could play a role, too. There’s some evidence from Israel that the Pfizer/BioNTech vaccine, at least, might be slightly less effective for people over the age of 80. Though on this question on effectiveness and age, Dean says “we don’t have a ton of data.”
It’s also the case that the vaccines just take a little while to reach full effectiveness. “The wisdom is that 14 days or some period after the second dose, you’re maximally protected,” Dean says. So you might be more likely to see a breakthrough infection early after being vaccinated, or when you’ve only received one dose of a two-dose vaccine.
Breakthrough infections are less severe, and possibly less transmissible — even with new variants
Here’s the good news: There are a lot of reasons to believe that when these breakthrough infections do occur, they’re not as bad as they would have been had the person not been vaccinated. For one, we know this from the clinical trial data of tens of thousands of people: Across all three approved vaccines, there was 100 percent efficacy in preventing deaths and hospitalizations. (Again, as Dean says, nothing is 100 percent. But this is pretty damn good.)
Data collected after the trials, in real-world conditions, is bearing this out, too. Recently, a 5000-patient study out of Israel compared cases of breakthrough infections (after vaccination) with infections that occurred among the unvaccinated. Simply put, the study found that those who had breakout infections also had smaller quantities of virus in them. This particular study didn’t include information on symptom severity. But lower viral loads have been correlated with lower disease severity; they also lower the risk of transmission to other people.
The Israeli study only assessed people who had taken the Pfizer/BioNtech vaccine (it’s pretty much the only vaccine being offered in Israel). But it stands to reason to expect a similar pattern from the Moderna vaccine and, perhaps, the Johnson & Johnson vaccine. The results of the study “should encourage people to get vaccinated,” says Idan Yelin, a biology researcher at Technion-Israel Institute of Technology, who co-authored the analysis. “When you get vaccinated, you do something good to yourself, but you also do good for people around you. That should be the message.”
But what about variants? Another study, yet to be published, also out of Israel, asked this question. It found that after one dose of the Pfizer/BioNtech vaccine, people were slightly more susceptible to infection with the B.1.1.7 variant (which was first discovered in the UK and is now the dominant strain in the US), and the B.1351 variant (which was first discovered in South Africa) than to the original SARS-CoV-2 virus. After the full two doses, the vaccines seemed to near-fully protect people against B.1.1.7 but were slightly less — but still quite — effective against B.1351.
Interestingly, the study authors note, even though they see the B.1351 variant is slightly more likely to break through, there’s just not a lot of B.1351 spreading in Israel. “We think that this reduced effectiveness occurs only in a short window of time, and that the S.A. variant [B.1351] does not spread efficiently,” Adi Stern, the senior author on the paper, tweeted.
That there isn’t much spread of B.1351 in Israel suggests “that these breakthrough infections are a dead end,” virologist Angela Rasmussen explains on Twitter. Though it might have a harder time spreading in Israel compared to other countries: To date, Israel has vaccinated more than 60 percent of its population.
Other vaccines may be less protective against variant breakthroughs, particularly the one produced by AstraZeneca: It might be only 70 percent effective against the B.1.1.7 variant, and just 10 percent effective against B.1351.
But the bottom line is still the same: The best protection against infections, from variants or not, is being fully vaccinated.
The number of breakthrough infections depends on the amount of virus circulating
If a vaccine is 95 percent effective, it means a person who is vaccinated is 95 percent less likely to get sick than another person, if exposed to the virus. The most important part of that last sentence: if exposed to the virus.
Breakthrough infections will occur in a small proportion of those who have been vaccinated, but the number of breakthrough infections that accumulate depends on how much virus is out there circulating in the community.
“The risk of these breakthroughs is really reflecting just how much transmission is in the community,” Dean says. Measles vaccines are not perfectly effective, either (they’re about 97 percent effective). But we don’t see a lot of measles cases among the vaccinated because it’s very rare to be exposed to the measles.
Simply put, the more cases there are, the more breakthrough infections there will be. So the answer to the problem of breakthrough infections is just to vaccinate more people (to lower transmission even further), continue to wear masks, and maintain distance among groups of people of mixed-vaccination status. (Read the CDC’s full guidelines for vaccinated individuals here.)
“Until we get these daily cases down, we’re going to be looking at these rare events,” Bromage says.