What We Know About Covid Vaccines and Delta

 https://elemental.medium.com/what-we-know-about-covid-vaccines-and-delta-d2114d758b0f

Fully vaccinated people aren’t likely to get super sick from delta, but they can probably get mildly sick and pass it on to others

 

he past two weeks have felt confusing, frustrating, and sudden for many people. As Covid-19 infection rates declined more and more throughout the summer, things started to feel a bit more normal-ish for many people.

Some felt like the pandemic was finally ending despite public health experts warning that it wasn’t and that another wave would arrive by fall. And then — BAM! — seemingly out of nowhere, the delta variant hit hard and fast. It’s now responsible for 83% of all infections in the U.S.

Now, just as families and schools are preparing for the upcoming school year, people are scrambling for answers, starting with the biggest one: How effective are Covid-19 vaccines against the delta variant? People who have been vaccinated and had begun feeling comfortable going maskless to stores, restaurants, and other places are now less certain about how protected they are. So let’s review the data we have so far.

Key takeaways

• The mRNA vaccines (Pfizer and Moderna) are extremely effective at preventing severe disease and hospitalization from the delta variant.
• The mRNA vaccines are less effective at preventing infection than disease, but the numbers vary on how much.
• The Johnson & Johnson vaccine is much less effective against infection with delta, but it protects well against severe disease.
• The rate of breakthrough infections in vaccinated people will be higher with delta than with past variants.
• We don’t yet have good data on the risk of vaccinated people transmitting delta to others, but most likely, vaccinated people can develop infections and pass the virus to others, vaccinated or not. In other words, if you’re fully vaccinated, assume it’s possible for you to get delta and give it to others.
• Immunity from natural infection (before delta) seems to protect almost but not quite as well against severe disease as the vaccines, but it probably doesn’t protect well against infection or transmission. In other words, if you have immunity from a past infection, assume it’s possible for you to get delta and give it to others.

What exactly is vaccine effectiveness?

First, it’s important to review what vaccine effectiveness actually means, especially since it means different things in different contexts. At the most basic level, vaccine effectiveness refers to the reduced risk when you divide the risk among vaccinated people by the risk among unvaccinated people. But that hinges on a key question: the risk of what?

Vaccine effectiveness can refer to the risk of infection, disease, severe disease, hospitalization, or death, and each of these is different. Most of the differences are obvious, but many people don’t realize there’s a difference between infection and disease. Infection refers to whether a virus takes hold in a person at all after they’ve been exposed. That is, does it begin to replicate in the person’s body, or do the person’s immune defenses prevent the virus from doing anything at all once it’s in the body? Disease refers to the symptoms that develop as a result of the virus replicating in the body, causing both an immune response and damage to the body.

All vaccines reduce the risk of disease. Not all vaccines reduce the risk of infection. What matters to most people is whether a vaccine prevents disease. But during a pandemic, some people care whether a vaccine prevents infection as well. The reason: If you have an infection, you may be able to transmit the virus to others, even if you don’t have the disease.

How effective are the Covid vaccines in general?

All three of the Covid vaccines authorized by the Food and Drug Administration were assessed according to how well they prevented severe disease, and they do that really well. If you intentionally, directly exposed 100 vaccinated people and 100 unvaccinated people to the SARS-CoV-2 virus that was circulating during the vaccine trials, and all 100 of the unvaccinated ones got severe disease, then only about five of the vaccinated ones would be expected to get severe disease. Severe disease has a very specific definition from the National Institutes of Health, which includes having a blood oxygen level below 94% and needing to take at least 30 breaths per minute.

Within a few months after the vaccines’ authorization, we learned some exciting news: The mRNA vaccines were also remarkably effective at preventing infection. They didn’t prevent 100% of infections, just as they didn’t prevent 100% of severe disease. But they prevented enough infections — somewhere between 75% to 90% — for experts to comfortably announce that the mRNA largely prevented transmission. That is, people vaccinated against Covid-19 were unlikely to contract the virus and pass the infection on to others, with or without symptoms.

Hence the Center for Disease Control and Prevention’s decision in May to say vaccinated people didn’t need to wear masks; they weren’t likely to spew the virus to those around them. Those studies, however, occurred while mask-wearing was still the norm, especially in hospitals, where most of those studies took place. Since then, an even larger study, involving more than 54,000 people from the Department of Veterans Affairs, found that the Pfizer vaccine prevented 96% of infections, and the Moderna vaccine prevented 98%. That study’s time period was also during universal masking. Still, those results are incredible.

But, that study also only looked at infections through March 2021. All that data we have on transmission prevention refers to the primary variant that dominated early in 2021 (called D614G) or to alpha, the B.1.1.7 variant from the U.K. Then along comes delta, which has the ability to bypass some of the protection from antibodies, whether from vaccines or prior infections. Now the distinction between vaccines preventing disease and vaccines preventing infection becomes much more important.

How well do the Covid vaccines prevent severe disease and hospitalization from delta?

You may have already heard repeatedly from national health experts, including Anthony Fauci, CDC director Rachelle Walensky, and U.S. Surgeon General Vivek Murthy, that the Covid vaccines are highly effective against the delta variant. That’s true, as we’ll see in the data in a moment. However, when these public health experts discuss how effective these vaccines are against delta variant infection, they are most often referring to how well the vaccine prevents severe disease — not how well the vaccine prevents infection.

Though the detailed data aren’t yet published, the Israeli health ministry recently reported that the Pfizer vaccine reduces the risk of severe disease by 91% and reduces the risk of hospitalization by 88%. In other words, if 100 vaccinated people and 100 unvaccinated people were all intentionally, directly exposed to the SARS-CoV-2 delta variant, and all the 100 unvaccinated people developed severe disease, only nine of the vaccinated people would be expected to develop severe disease. (Not all 100 unvaccinated people would actually develop severe disease. I’ve used these numbers to illustrate how relative risk reduction with vaccine effectiveness works.) However, we don’t know how many people these findings are based on or how the ministry collected the data.

A recent study, published July 21 in the New England Journal of Medicine, looked at how well the Pfizer and AstraZeneca vaccines prevent any disease at all, whether mild, moderate, or severe. The study involved 19,109 Covid cases, including 4,272 delta cases. The rest were alpha.

• The Pfizer vaccine prevented 94% of disease from alpha and 88% of disease from delta.
• The AstraZeneca vaccine prevented 75% of disease from alpha and 67% of disease from delta.

This study did not assess the Moderna vaccine or Johnson & Johnson vaccine. However, the Moderna vaccine is similar enough to the Pfizer one that we can reasonably presume the effectiveness of both will be nearly the same. The Johnson & Johnson and AstraZeneca vaccines aren’t quite as close to one another as Moderna and Pfizer, but they’re still quite similar. It’s likely the numbers for Johnson & Johnson would be very close to those for AstraZeneca.

Bottom line: If the delta variant virus entered the body of 100 people fully vaccinated with an mRNA vaccine, 12 of them would likely develop disease (though mostly mild).

How well do the Covid vaccines prevent infection from delta?

In a recent study from Scotland, researchers calculated how well two vaccines prevented infection. The study period was April 1 to June 6, 2021, when 97% of non-alpha-variant infections were delta infections. The study looked at more than 19,500 infections, including 7,723 delta infections. The rest were alpha.

• The Pfizer vaccine prevented 92% of alpha infections and 79% of delta infections.
• The AstraZeneca vaccine (not used in the U.S.) prevented 73% of alpha infections and 60% of delta infections.

Bottom line: The researchers found that the Pfizer and AstraZeneca vaccines did reduce infection risk but not as well as they did for past variants.

However, reports from the Israeli health ministry suggest the Pfizer vaccine is even less effective in preventing infection than the Scottish data suggests. They reported the Pfizer vaccine prevented only 39% of infections from delta. (Since they said the vaccine prevented 91% of severe disease, that means half the infections in vaccinated people were either asymptomatic or involved only mild or moderate disease.) Again, though, we don’t know how many cases this is based on, and we haven’t seen the details of the data.

We don’t have enough information to know just how much less effective the mRNA vaccines are at preventing infection with delta, but one thing is very clear: There will definitely be more breakthrough infections in vaccinated people with delta than past variants.

What about the Johnson & Johnson vaccine and delta?

Although there are similarities between the AstraZeneca and Johnson & Johnson vaccines, we can’t assume the data from AstraZeneca vaccines will be the same in J&J ones, especially since the AstraZeneca vaccine involves two doses, and Johnson & Johnson is a one-dose vaccine.

Johnson & Johnson reported in early July that its vaccine protected against delta, but it didn’t provide a percentage or data from actual cases. Instead, it tested antibodies from eight people vaccinated with Johnson & Johnson against delta in a lab and found “neutralizing activity” from the antibodies against delta. That means the antibodies did attack some of the delta strain, but we don’t know how much or if it was enough to prevent disease. (Moderna reported similarly nonspecific data on neutralizing antibodies for its vaccine against delta.) A separate Johnson & Johnson study with 20 people also showed neutralizing activity against delta, but again, we have no idea how that translates to cases in the real world.

A newer study suggests the Johnson & Johnson vaccine is substantially less effective at preventing disease from delta. Researchers from New York University tested antibodies created by the Johnson & Johnson vaccine in the lab and found them to be about five times weaker against delta than the original variant. The same study found the Pfizer and Moderna vaccine antibodies to be about three times weaker against delta. This still isn’t real-world data, which would be more helpful in understanding how well the Johnson & Johnson vaccine actually protects against delta, but it does give us a way to compare it to the mRNA vaccines. So far, there’s no evidence to suggest more severe disease occurs in people vaccinated with Johnson & Johnson, which is good.

So if I’m vaccinated and I get infected with delta, can I pass the virus on to others?

That is the million-dollar question. The shortest answer: very likely yes. The CDC is currently banking on the answer being no—that’s why it hasn’t changed its masking advice despite plenty of smart public health folks saying they should. (Note: Hours after I posted this article, the CDC announced a briefing in which it changed its mask guidance, recommending indoor masking for everyone, including vaccinated people, in areas with high levels of transmission, meaning 50+ cases per 100,000 people in the past week. It also recommended indoor masking for all vaccinated people who are immune-compromised or at high risk for severe disease or who live with someone immune-compromised, at high risk for severe disease, or unvaccinated. Finally, they recommended universal indoor masking for all teachers, staff, students, and visitors to schools, regardless of vaccination status, bringing their recommendations in line with those of the American Academy of Pediatrics.)

But plenty of anecdotal evidence points to yes, vaccinated people infected with delta can infect others, and the precautionary principle — erring on the safe side — suggests it’s better to assume you can infect others if you’re sick with delta, vaccinated or not. That means everyone wearing a mask protects other people, and, contrary to current CDC guidelines, if you’re vaccinated and get exposed to Covid, it’s socially responsible to get tested and quarantine yourself until you’re certain you’re not infected.

Why did the CDC and other U.S. health officials say you probably wouldn’t infect others if you’re vaccinated and catch delta? We know that viral load — the number of virus particles in your body — correlates with how contagious you are. The more virus particles you have in your body, the more you’re shedding to the environment around you, the more contagious you are. We also know that the viral load is much lower in vaccinated people who get infections. A study in February found that people who received just one dose of the Pfizer vaccine and got infected had a viral load 20 times lower than unvaccinated people with infections. So, if you’re vaccinated and get infected but you have far, far fewer virus particles in your body (which is also why you’ll have few symptoms or no symptoms), you should be less contagious.

Again, however, that was true before delta came along. The problem with the CDC’s current logic (Note: that is, before it changed its guidance on July 27) is that it based the guidance on data for past variants. A recent study, based on 62 cases, found the viral load in people with delta infections is about 1,000 times greater than the viral load in people with pre-delta infections. (Virologist Angie Rasmussen tweeted a helpful analysis of that study.) It also found people tested positive sooner: four days after exposure instead of six. That means people are shedding more virus and doing it sooner than with infections from past variants.

If viral loads are 1,000 times greater in unvaccinated people, there’s a reasonable chance they’re greater in vaccinated people as well. (Note: In its briefing, the CDC noted recent research that found that the viral load in vaccinated people with a delta infection was similar to the viral load in unvaccinated people with a delta infection.) The combination of a higher viral load, less protection against infection, and a faster incubation period (time from exposure to time of first symptoms) means vaccinated people likely have an excellent chance of passing the virus on to others.

And what about protection from natural infection against delta?

Some people have chosen not to get vaccinated because they feel they have enough immunity against the virus from a past infection. Most likely, however, their antibodies are even less effective against delta than those from the vaccines.

The same study that tested antibodies from the Johnson & Johnson vaccine in the lab against delta also tested antibodies from people who were infected before the variants arrived. They found the antibodies from past infection were three to five times weaker against the delta, beta (B.1.351, first identified in South Africa), and lambda (C.37, first identified in South America) variants. The same study found the antibodies from mRNA vaccines were two-and-a-half to four times weaker against delta, beta, and Llmbda, which means the antibodies from the vaccine were slightly stronger than those from natural infection.

Again, bottom line: If you had a Covid infection before delta arrived, you are still susceptible to a delta infection and passing that infection on to others.