How Good Are Covid-19 Vaccines At Protecting Against The Delta Variant?

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Covid-19 cases are once again on the rise in certain parts of the United States driven by the more contagious Delta variant. In just a few short weeks Delta has become the dominant variant in the country, and now makes up 83% of new cases in the United States. While the vast majority of cases are among unvaccinated people, there have also been some “breakthrough” cases, resulting in a small number of hospitalizations and deaths among people who were fully vaccinated. These cases are few and far between and experts say the vaccines are still working as intended, even against Delta. Here’s what we know about how vaccines work against this new variant. 

Is the Delta variant more dangerous than previous variants?

As of now, there is no evidence that the Delta variant makes people more severely sick, but it is around twice as contagious as the original SARS-CoV-2 virus. This is in part because it’s better at hiding from the body’s immune system through mutations on the spike protein which help it evade detection by the army of virus-fighting antibodies produced by the vaccine. It is also more contagious because people tend to carry around more particles of the virus in their nose and throat, making it easier to transmit to others. Finally, there is some evidence that it has a shorter incubation period, which means people get infected faster and so it’s harder to track cases before more infections occur. 

How can I still get infected if I’ve already been vaccinated?

“The vaccine is not a bug zapper,” explains Amesh Adalja, a physician and senior scholar at the John Hopkins Center for Health Security. What he means is that at first the virus isn’t completely neutralized like a squashed bug. When the SARS-CoV-2 virus enters the body of a vaccinated person it takes some time before the immune system is alerted to the intruder and antibodies spring into action. While the virus can enter the body, the immune response generated by vaccination means it usually can’t set up shop in cells and replicate as it would in an unvaccinated person. “The virus is circulating out there in the community amongst people and you’re going to come into contact with it,” he says. “What we want the vaccine to really do is prevent serious disease, hospitalization and death. Everything else is kind of gravy.”

Do breakthrough infections mean the vaccines aren’t good enough? 

In short, no. A breakthrough infection occurs when someone who has been fully vaccinated tests positive for Covid-19. The good news is breakthrough infections are rare and the vast majority of cases are mild. More than 161 million people in the U.S. have been fully vaccinated, and fewer than 6,000 fully vaccinated people have been hospitalized or died from breakthrough infections. The CDC has stopped collecting data on asymptomatic breakthrough infections, meaning people who have tested positive but don’t have any symptoms. “The fact that people are getting breakthroughs with the Delta variant and not having symptoms is something to celebrate,” says Adalja. “That means our vaccines are robust against the Delta variant.” People who do get more severe breakthrough infections are likely to be elderly or immunocompromised. Seventy-five percent of people who had severe breakthrough infections were over the age of 65. It is still extremely rare to get severe Covid-19 after being vaccinated, especially if you are young and healthy.

How effective are the vaccines against Delta, exactly? 

While some vaccines are slightly less effective against the Delta variant compared to the original SARS-CoV-2 virus, all of the vaccines authorized for use in the U.S. are still very effective against the variant. If you do get Delta while vaccinated, you are much less likely to get severe disease, be hospitalized or die. 

The Pfizer/BioNTech vaccine is 88% effective against Delta after two shots versus 93.7% against original Covid-19 after two shots, according to a lab study. What this means is that if everyone in the U.S. gets vaccinated, the country will experience 88% fewer Delta cases compared to if no one was vaccinated. This does not mean that 88 out of 100 people would be protected from Delta. But after only one shot, the vaccine is not very effective at protecting against the Delta variant. Some other studies have found conflicting results, like a study done in Israel that found the Pfizer/BioNTech vaccine was only 64% effective against Delta. 

A lab study also found that the Moderna vaccine was 72% effective against the Delta variant after one dose, but more studies are needed to understand how protective it is. More studies are also needed for the Johnson & Johnson vaccine. One study that has not yet been peer reviewed suggests the Johnson & Johnson vaccine is less effective against the Delta variant, but that isn’t enough information to make a definitive conclusion. 

Interestingly, some people in other countries have been finding success with “mixing and matching” Covid-19 vaccines. A recent study in Korea found that taking one dose of the AstraZeneca vaccine followed by one dose of the Pfizer vaccine produced more antibodies than two doses of the AstraZeneca vaccine. It is unknown whether public health experts might recommend mixing and matching vaccines in an effort to fight the Delta variant. 

Are we going to need booster shots to protect against Delta?

Maybe, depending on the vaccine. Pfizer and BioNTech say that an ongoing analysis shows the vaccine’s efficacy wanes over time and a booster shot may be needed. According to a Pfizer spokesperson, “Pfizer and BioNTech have put into place a robust booster research program to ensure that our vaccine continues to offer the highest degree of protection possible.” Plus, if additional studies find a booster shot is not effective against the Delta variant, Pfizer/BioNTech “expect to be able to develop and produce a tailor-made vaccine against that variant in approximately 100 days after a decision to do so,” according to a press release. Right now, there is not enough data to determine whether people who were vaccinated with the Johnson & Johnson or Moderna vaccines will need a booster shot.



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