Why have been fully vaccinated but still infected?

A journalist reader wanted me to comment on the idea that because there are vaccines with 50-60% effectiveness, some people who are fully vaccinated can still infect others. I would like an alternative explanation involving 5 factors related to vaccine type, duration, viral variant and clinical history.

The ‘phenomenon’ of being infected with the virus after vaccination (full 2 ​​doses) is not new. In epidemiology, they are called ‘breakthrough infections’.

Breakthrough infections are uncommon. According to a British study, 1 out of every 500 fully vaccinated people will be infected. Therefore, you can say that the probability that you have been vaccinated with nCov is about 0.2%, which is low. In the US, CDC data shows this rate to be 0.01%.

May be an image of text that says 'According to the CDC, there were: 10,262 breakthrough SARS-CoV-2 CoV-2 infections in the US among the first 101 million fully vaccinated people.  Healio'
Data from the CDC (as of May 25, 2021) of 101 million people vaccinated, only 10262 people (or 0.01%) had a ‘breakthrough infection’.

But not everyone has the same probability of a breakthrough infection. Some people are at high risk, some are at low risk. (Please add that I use the word ‘danger’ or risk here is synonymous with ‘probability’). So the question is what factors can identify people at high or low risk of infection? Yes, yes, and I summarize those factors as follows:

Factor 1: type of vaccine

We already know that there are many different types and formulations of vaccines against covid. Not only that, the effectiveness of vaccines can also vary. If based on clinical trials, the vaccine effectiveness of Pfizer is 95%, Moderna 94%, AstraZeneca 70-81% (depending on injection time). As for the Chinese vaccine, they report as effective as AstraZeneca, but in reality it seems to be lower, so people still need to get more Western vaccines.

Please add that when it comes to ‘95% efficiency’ ARE NOT This means that for every 100 people who inject it, 95 people will not be infected. Completely not right. It means that the probability of infection in vaccinated people is less than 95% of the probability in unvaccinated people.

The metric for efficiency is probability, not individuality. Well, probability applies to a group of people, not to an individual. A group of people will have infected and uninfected people. The individual is only infected or not infected, not the probability of infection. This means that the vaccine effectiveness figure only applies to a population or group of people, it has no practical significance for an individual.

Factor 2: time between 2 doses

According to the results of the AstraZeneca vaccine study reported on Lancet [1], the time interval at which the vaccine is most effective is about 3 months. Experts explain that 3 months is enough time for our body to ‘get used to’ the vaccine before receiving a new dose. You can read the chart that I quoted below to see about 12 weeks is optimal. When the interval between 2 doses is 12 weeks, the vaccine effectiveness is up to 81%, but when the interval is 6 weeks, it is only 55% effective.

Relationship between time between 2 doses of vaccine and efficacy of AstraZeneca vaccine. The period of 3 months or more is more effective than the period of less than 3 months.

That is also the reason that Australia chooses a 3-month interval to achieve the highest efficiency. But in places where vaccines are rare (like Vietnam, for example) authorities tend to shorten the vaccination time between two doses.

Factor 3: time of effect

No vaccine works ‘forever’, but only for a while. According to the (unreviewed) study, the effectiveness of the Pfizer vaccine seems to decrease after 6 months after the second dose. [2]. Jewish side studies [3] also came to the same conclusion.

Factor 4: variant of the virus

The fourth reason is because the virus has a variant that helps it escape the control of the immune system. The genetic material of the nCov virus is RNA (different from humans is DNA). RNA has a very, very rapid mutation rate than DNA. When we have a vaccine to fight them, they have already mutated into another form, because they are often far ahead of humans.

This may explain why the new mutated virus helps them escape the radar of the immune system and freely attack people. That’s why scientists were concerned when India discovered a new variant of the Wuhan virus, because it could make the existing vaccine less effective.

Factor 5: age and health

The fifth reason is age, health status, and especially drug history. Compared to young people, ‘old’ immune systems (like mine and my peers) don’t respond well to new antigens. (Antigens are external factors that cause our immune system to produce antibodies to fight viruses.)

This may sound like a joke, but the truth is, there are studies that explain the correlation between age and immune response in people vaccinated with the Pfizer vaccine. [4]. Therefore, my guess is that the breakout cases may be, on average, older and healthier than those without the breakout.

Summary, Looking at the literature, I think the reason for getting infected even though 2 doses of the vaccine have been given is (a) the type of vaccine; (b) the time interval between 2 doses is not sufficient to optimize the effectiveness of the vaccine; (c) vaccine effectiveness declines over time; (d) a variant of the virus; and (e) age-related factors and comorbidities.

Whatever the explanation, we must be aware that with a full 2-dose vaccination we still have a risk of infection (although the risk is very low). It should be remembered that vaccines are important, but not the ‘silver bullet’ against covid-19 that WHO experts have warned. Public health measures (such as restrictions on mass gatherings) remain in place for some time.


[1] https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)00432-3/fulltext

[2] https://www.medrxiv.org/content/10.1101/2021.07.28.21261159v1

[3] https://www.medrxiv.org/content/10.1101/2021.08.24.21262423v1

[4] https://www.medrxiv.org/content/10.1101/2021.03.03.21251066v1.full

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