Covid -19 Vaccine Efficacy Numbers, Explained - The Comprehensive Minds

Covid -19 Vaccine Efficacy Numbers, Explained

Covid -19 Vaccine Efficacy Numbers

When Pfizer first reported the efficacy of their COVID-19 vaccine, a staggering 95%, many thought that would mark an end to the pandemic, but that hasn't happened. Vaccination rates have remained low in certain parts of the world, despite multiple available vaccines. 

And there's also been reports of breakthrough cases, where fully vaccinated people get COVID. And now, recent unpublished data from Israel showed, that the Pfizer vaccine was only 40% effective at reducing the risk of symptomatic COVID. 

unpublished data from Israel

So if the original trials showed high efficacy, why did a large portion of people in this Israeli study get COVID? Here's why those two numbers are so far apart and why neither of them can tell the full story. To understand this gap, first, you need to understand the difference between efficacy and effectiveness.


Vaccine efficacy refers to how well a vaccine performs under ideal, tightly controlled conditions, like clinical trials. For example, during trials for COVID-19, vaccines were given to people on a tight schedule. It also has to do with what the researchers were studying. 

Here, they were looking at how well vaccines prevented symptomatic infections in a limited number of people, not whether they prevented infection altogether, or kept people from being contagious. And with COVID-19, it's possible to be infected and not show symptoms. - What determines how efficacious, or how well the vaccine works in a trial, is really two things. 

Lisa Lee

Lisa Lee has studied the spread of viruses for 30 years and has held leadership roles at the CDC.  According to her, It's how contagious the virus is, how many people an infected person might infect in the absence of that vaccine. And it also depends a lot on the host, that is people. How well can the vaccine stimulate the right immune response for people? 

In Pfizer's study, the company tested about 44,000 people for symptomatic COVID-19, and only 170 develop the disease with at least one symptom. And of those, just eight had been vaccinated, while 162 had gotten the placebo. 

This is what added up to that 95% efficacy rate. This, by the way, means that in a vaccinated population, 95% fewer people will contract the disease when they come in contact with the virus, not that each individual is 95% immune. 

Also Read: Is a natural Covid infection really 13 times better than being vaccinated? 

But that efficacy rate was calculated during a specific time and under a specific set of circumstances. Many people were staying at home, avoiding large gatherings, wearing masks, and the virus hadn't mutated as much. 

Vaccine effectiveness, on the other hand, is how well a vaccine works in the real world, outside of clinical settings, where it's harder to control variables that were constant in a study, like when people get their vaccines. 

You know, the real world is kind of messy. Not everybody gets their vaccines at the exact interval. Not everybody has the same kind of immune system. And behavior is changing too. In many places, businesses are open, and large gatherings are happening again. 

This is part of why efficacy numbers in trials are often higher than the effectiveness we see in the real world. And what makes this all the more complicated is that the virus itself is still evolving. The Delta variant, which has become dominant in multiple countries, is more transmissible than earlier strains of the virus, which means that more people will come in contact with it. 

It became dominant, months after the vaccines had been authorized, and millions of doses had been distributed. That means our current shots were designed for an earlier version of the coronavirus and don't work as well against Delta. 

This is part of why we're hearing more about breakthrough cases. U.S. states counted over190,000 breakthrough cases between January 1st and early August this year. That's among millions who are already vaccinated. 

Israeli study data

And remember that Israeli study, data was collected for it in June and July, when Delta was dominant there. Health officials saw breakthrough cases were more likely among people who had gotten the vaccine early on, in January or February. 

This is partly because antibodies, which the body produces after the vaccine to protect itself against the virus, naturally decrease over time. 

That, along with the unknowns about variants, is a big reason why the Biden administration announced new vaccine recommendations. - They are announcing their plan to stay ahead of this virus, by being prepared to offer Covid-19 booster shots, to fully vaccinated adults, 18 years and older. - According to Lisa Lee, what we want to do is make sure that we keep that immunity high enough, so when it starts to wane, we want to make sure that people are able to get that third shot. 

That should increase the chances that the vaccines prevent severe illness that could then lead to hospitalization and death. Still, there is some debate among public health experts as to whether booster shots are needed right now, that's because antibodies are not the body's only defense against the virus.


There are also certain immune cells that prepare the body to fight off pathogens, and those may not be waning. And while breakthrough cases continued to occur, experts say that these cases are usually mild and don't result in severe illness. 

Israeli data that showed that the Pfizer vaccine

Let's go back to that Israeli data that showed that the Pfizer vaccine is now only 40% effective against symptomatic COVID. Well, that same study showed that the vaccine had a 91% effectiveness against hospitalization and death. 

The vaccine really, what it does is help us slow the speeding car, and it helps us to reduce transmission. And that is what's gonna get us out of this pandemic. In addition to vaccines, experts also recommend preventive measures like mask-wearing and social distancing to keep the virus from spreading and mutating further. 

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