About two weeks after you’re vaccinated, your anti-corona antibody level is at its peak and you’re considered fully vaccinated.
But, as some people discovered in the fourth wave of the pandemic, even two full doses of the vaccine don’t guarantee 100% protection against infection. A breakthrough infection looks quite similar to being normally infected with corona, but there are important differences.
A study of corona symptoms shows that the five most common symptoms among vaccinated people infected with corona are: headache, runny nose, sneezing, sore throat and impaired sense of smell.
This sounds similar to the usual and familiar symptoms experienced by the unvaccinated.
But without the vaccine there are two other symptoms from which the vaccinated are spared: fever and persistent cough.
One study comparing symptoms among people vaccinated and unvaccinated found that a vaccinated person infected with corona had a 58% less chance of developing a fever. Most people who were vaccinated and then infected said they felt more like they had a minor cold.
Another difference is the risk of being hospitalized.
The vaccinated have a much lower chance of being hospitalized because of their symptoms and are less likely to suffer long-term symptoms, often referred to as long-COVID.
What increases the risk of infection in vaccinated people?
A study in the United Kingdom found that the infection rate of the vaccinated stands at 0.2%, which means that one in 500 people is infected even though they’re vaccinated.
However, the chance of getting infected despite the vaccine isn’t equal for everyone, and there are four factors that increase risk.
1 - Type of vaccine
The clinical studies conducted prior to the approval of the various vaccines, as well as the data collected after the start of the global population vaccination campaign, gave each of the approved vaccines a score regarding the relative reduction of infection.
For Pfizer, most common among Israelis, the vaccine is 95% effective in reducing infection. Moderna’s vaccine is very close, with 94% efficiency.
Other vaccines that have been approved and administered worldwide have presented less effective data: the Johnson & Johnson vaccine was 66% and the Astra-Zenica vaccine 70%. So someone who has been vaccinated with one of the last two has slightly less effective protection than someone vaccinated with Pfizer or Moderna.
2 - Time elapsed since last dose was received
The length of time that elapses since the vaccine is given turns out to be one of the most significant factors on the level of antibodies and hence also on the quality of the immunization. This is why Israel decided to vaccinate the population with a third booster dose and it seems that the rest of the world will follow suit.
The United States has already started giving the booster to some parts of the population, and so have several European countries.
Recent studies indicate that immune resistance as a result of Pfizer vaccines subsides during the first four to six months after the second vaccine.
3 - Variants
The virus we’re dealing with today isn’t exactly the same one that burst into our lives almost two years ago. Rather, there are new and sometimes slightly upgraded versions of it.
Certain mutations that the virus underwent during its worldwide travels brought us a variety of variants, which are now being named with Latin letters of the alphabet. The Delta variant and its descendants have become the most dominant strain of the virus in most of the world.
The efficacy percentages of the vaccines were calculated at the experimental stage relative to the original version of the virus. However, by the time they were approved and administered, the vaccines were being asked to defend against slightly different versions of the virus. Thus, the efficacy percentages of Pfizer's vaccine, which stood at 95% against the original virus, dropped slightly to 93% against the Alpha variant and to 88% against Delta.
As such, the variant to which you’ve been exposed, which you usually don’t know, will also affect your chances of getting infected even though you’re vaccinated.
4 - Your immune system
All the data mentioned in this article and in the studies it cites refer to the average risk relative to the general population. It’s important to remember that each of us is an individual case, and the risk of each person to be infected is the result of a complicated equation that consists of the type of vaccine received, how many doses were taken and when, and a person’s chances of contracting the virus based on their lifestyle. For example, medical workers or teachers are often in situations where their chances of contracting the virus are higher.
But the strength of an individual’s immune system also plays a role.
The strength of the immune system usually decreases with age. In addition, chronic or long-term medical conditions can weaken the immune response. This is why the elderly and those who are immunosuppressed tend to develop fewer antibodies after being vaccinated and are considered more vulnerable to infection.
In most countries these groups have been given priority in vaccination, which means they have been vaccinated in the earlier stages to protect them. But it also means that their immune defenses start to go down sooner.