The U.S. CDC has released some information on breakthrough COVID-19 infections on people who have been fully vaccinated. The bad news: 5,800 people have gotten the coronavirus in spite of the vaccine. The good news: that’s out of more than 66 million people, for a breakthrough rate of 0.008%.
What do we know about these breakthrough infections?
The information we currently have is from this Wall Street Journal article. The CDC hinted they will release more information next week.
Of the breakthrough infections, 29% were asymptomatic. Only 7% resulted in hospitalisation, and only 74 people (1.2% of breakthrough infections, or 0.0001% of the people who were fully vaccinated) died of COVID.
Those are excellent numbers. They show that the vaccine is highly effective, and may support the idea that even if you get COVID after being vaccinated, you’ll likely get a milder case than if you hadn’t been vaccinated. (This is true of the flu shot as well.)
How does this compare to the efficacy rates (for example, 95% for Pfizer) from clinical trials?
It’s a different calculation, so the numbers don’t compare directly. But the two are compatible.
In clinical trials, one group of people gets the real vaccine and another group gets a placebo shot that does not contain the vaccine. Over time, as people live their lives, some will come down with COVID. Investigators compare the risk of getting COVID in the vaccination group versus the placebo group. That’s your trial efficacy number.
2) Vaccine efficacy (VE) measures the relative reduction in infection/disease for the vaccinated arm versus the unvaccinated arm. A perfect vaccine would eliminate risk entirely, so VE = 1 or 100%. This can be calculated from the risk ratio, incidence rate ratio, or hazard ratio. pic.twitter.com/luaf70aOZE
— Natalie E. Dean, PhD (@nataliexdean) September 28, 2020
This is different from real-world effectiveness for a few reasons. One is that the trial population might not be the same as the people who get the vaccine in the real world: different age ranges, medical histories, and so on.
Another is that the virus may be more or less common in the world over time. If case counts go down overall, we would see fewer breakthrough infections just because there is less virus going around.
Bottom line, both the trial efficacy and the new data on breakthrough infections tell us that the vaccine works really well.
How can a person get infected if they’re vaccinated?
When we’re talking about millions of people, there will always be a few cases that don’t go the way we expect. Every vaccine has an effectiveness that you can look up. For example, the measles vaccine is 97% effective and the mumps vaccine is about 88% effective. The flu shot’s effectiveness varies by year, ranging usually between 40% and 60%.
Why not 100%? Well, a few things can happen. One is that not everybody’s immune system reacts properly to the vaccine in the first place. People whose immune system is weak to begin with may not respond as well; this is why a high-dose flu shot is recommended for people over 65.
Another possibility is that a vaccinated person can sometimes still get sick when they are exposed to an amount of the virus that overwhelms their immune system. This could possibly be from a large infectious dose (for example, somebody very infectious coughing in your face all day) and potentially from other health conditions that weaken your immune system.
And finally, another possibility is that the virus may have found a way to evade a well-prepared immune system. If one of the new coronavirus variants has a spike protein that is different enough from the one encoded by the vaccine, your immune system may not recognise the new invader. That’s why the variants are so concerning, and why many public health experts are advising that we continue to wear masks in public places even when we are vaccinated.
The CDC is gathering information on breakthrough infections to try to determine what they have in common: are some people more prone to these infections than others? Are the breakthrough infections mostly from a certain strain or variant of the coronavirus? We don’t know the answers yet, but perhaps we will soon.