Progressive Covid-19 infections are possible after vaccination. But do not worry.

Lately there have been some scary sounding headlines when it comes to getting sick with Covid-19 after being vaccinated:

The point is, none of these headlines should surprise or scare us that much.

Based on clinical trial data, the Pfizer / BioNtech vaccine is 95 percent effective in preventing Covid-19 disease. The Moderna vaccine is 94 percent. Johnson & Johnson’s one-dose vaccine is 66 to 72 percent effective (and highest at preventing serious disease). What you’ll notice in all of those figures: None are 100 percent effective in preventing disease.

This should be obvious from clinical trial data, but it does surprise some: 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 hope to see: some level of cases among vaccinated people,” says Natalie Dean, a biostatistician at the University of Florida.

It is also not surprising to see some (extremely rare) cases of hospitalizations and serious illness among people who have been vaccinated. That may seem even more puzzling since clinical trials of tens of thousands of people reported that these vaccines were 100 percent effective in preventing hospitalizations.

But “nothing is 100 percent,” says Dean. “When you start talking about vaccinating millions of people, even things that happen relatively infrequently will start to show up.” The real culprit for these infections is not vaccines, it is the fact that this pandemic is still wreaking havoc in many communities.

But why would a vaccinated person ever get sick?

Until now, I have been talking about the efficacy of the vaccine in terms of disease prevention. There is also the question of vaccines that prevent infection. It is a little different: the disease shows symptoms; The infection is simply a positive test for the virus (perhaps asymptomatically). On this, so far, it seems that vaccines are also very good at preventing infections. A recent Centers for Disease Control and Prevention study of 3,950 healthcare workers found that mRNA vaccines (Moderna and Pfizer / BioNtech) were 90 percent effective in preventing any infection.

There is less real-world data on the Johnson & Johnson vaccine, but the CDC writes that preliminary evidence suggests that it also “might provide protection against asymptomatic infection.”

Which is all to say: Most of the time, these vaccines will prevent Covid-19 infection from taking hold in the body. Cool! And that’s why these vaccines are our best chance to end the pandemic. But Some Most of the time, it will not prevent infection and it will not prevent symptoms.

Why? Think of the immunity the vaccine confers as prey, says Erin Bromage, an immunologist at UMass Dartmouth.

“No dam, for example, is 100 percent effective,” he says. “They are designed to handle 20-year storms, 50-year storms. But they are overwhelmed when a 100-year storm hits. ”

But there are circumstances in which infection can overwhelm this immune prey.

The exact reasons are not precisely known. Some of the circumstances that lead to breakthrough “can be predicted, but others simply cannot be,” says Bromage. “You don’t know who it will be.”

Just as people have variable immune reactions and variable severity of illness when they contract the virus, people have variable (but less) immune reactions to vaccines. (It’s also how some people get a lot of side effects from the vaccine and others don’t. Everyone’s body responds a little differently.)

Therefore, based on what we know so far, it is difficult to predict a large portion of who will experience serious infections.

But there are also some things that scientists suspect could make it more likely.

One could be the amount of virus a person is exposed to. Much like when a dam breaks when a major flood hits, the immune protection of vaccines could be overwhelmed when a person is simply exposed to too many viral particles (perhaps living with someone sick with Covid-19 or working in a health center). health dealing with Covid-19 patents).

Other times, “the foundations on which the dam was built is not strong enough,” Bromage says, causing the dam to break. An immunosuppressed person may be at increased risk for a breakthrough infection. “We know, for example, that people with HIV had lower levels of protection,” says Dean. “It may reflect something about the immune response.”

Age could also play a role. There is some evidence from Israel that the Pfizer / BioNTech vaccine might, at least, be slightly less effective for people over the age of 80. Although on this question about effectiveness and age, Dean says: “We don’t have a lot of data. “

And then there are more common things in our lives that can temporarily make us more susceptible to infection. The quality of sleep can influence the strength of the immune system. Stress can do it too.

It is also the case that vaccines take a while to reach maximum effectiveness. “The wisdom is that 14 days or some period after the second dose, you are fully protected,” says Dean. Therefore, you are more likely to see a breakthrough infection at the beginning of the vaccination or when you have only received one dose of a two-dose vaccine.

Breakthrough infections are less severe and possibly less communicable, even with newer variants.

Here’s the good news: There are many reasons to believe that when these infections do occur, they are not as bad as they would have been if the person had not been vaccinated. On the one hand, we know this from clinical trial data from tens of thousands of people: Across the 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 good.)

The data collected after the tests, under real-world conditions, also confirms this. Recently, a study of 5000 patients in Israel compared cases of breakthrough infections (after vaccination) with infections that occurred among the unvaccinated. Simply put, the study found that those who had progressive infections also had smaller amounts of the virus. This particular study did not include information on the severity of symptoms. But lower viral loads have been correlated with less disease severity; they also reduce the risk of transmission to other people.

The Israeli study only evaluated people who had taken the Pfizer / BioNtech vaccine (it is practically the only vaccine offered in Israel). But it is logical to expect a similar pattern from the Moderna vaccine and, perhaps, from the Johnson & Johnson vaccine. The results of the study “should encourage people to get vaccinated,” says Idan Yelin, a biology researcher at the Technion-Israel Institute of Technology, a co-author of the analysis. “When you get vaccinated, you do something good to yourself, but you also do good to the people around you. That should be the message. “

But what about the variants? Another study, yet to be published, also from 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 UK). USA), and variant B .1351 (first discovered in South Africa) than the original SARS-CoV-2 virus. After the two full doses, the vaccines appeared to almost completely protect people against B.1.1.7 but were slightly less, but still quite, effective against B.1351.

Interestingly, the study authors note that while they see that the B.1351 variant is more likely to break through, there simply isn’t much of a spread of B.1351 in Israel. “We believe that this reduction in efficacy occurs only in a short period of time, and that the SA variant [B.1351] it doesn’t spread efficiently, ”tweeted Adi Stern, the lead author of the article.

The fact that there is not much spread of B.1351 in Israel suggests that “these groundbreaking infections are a dead end,” says virologist Angela Rasmussen. explain on Twitter. Although it might have a more difficult 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 advances, particularly the one produced by AstraZeneca: it could be only 70 percent effective against variant B.1.1.7 and only 10 percent effective against B.1351.

But the bottom line remains the same: the best protection against infections, variant or not, is to be fully vaccinated.

The number of breakthrough infections depends on the amount of virus circulating.

If a vaccine is 95 percent effective, it means that a person who is vaccinated is 95 percent less likely to get sick than someone else if exposed to the virus. The most important part of that last sentence: if you are exposed to the virus.

Progressive infections will occur in a small proportion of those who have been vaccinated, but the number of progressive infections that accumulate depends on the amount of virus circulating in the community.

“The risk of these advances really reflects how much transmission there is in the community,” says Dean. Measles vaccines are also not perfectly effective (they are about 97 percent effective). But we don’t see many cases of measles among those vaccinated because it is very rare to be exposed to 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 simply to vaccinate more people (to further reduce transmission), continue to wear masks, and keep the distance between groups of people in mixed vaccination status. (Read the full CDC guidelines for vaccinated people here.)

“Until we get these daily cases, we will be looking at these rare events,” says Bromage.

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