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What increases the risk of breakthrough infection?

By on September 14, 2021

Although the coronavirus vaccination drive in Japan is in full swing, we hear rising cases of breakthrough infections that are causing some people to worry. A breakthrough infection is a case of illness in which a vaccinated person becomes sick from the same illness that the vaccine is meant to prevent.

According to the COVID Symptom Study, the five most common symptoms of a breakthrough infection are a headache, a runny nose, sneezing, a sore throat, and loss of smell. Some of these are the same symptoms that people who haven’t had a vaccine, experience. If you haven’t been vaccinated, three of the most common symptoms are also a headache, sore throat, and runny nose.

On August 17, Mainichi newspaper reported the Japanese government and U.S. pharmaceuticals giant Pfizer have broadly agreed to a contract supply for an additional 120 million booster doses for 2022. Given that there is lack of clarity on the duration of vaccines’ ability to prevent developing COVID-19 and serious cases of the disease, and the emergence of the more infectious delta variant, the Japanese government views that an additional “booster” shot is necessary.

According to a research, one in every 500 people in the UK or 0.2% of the population experiences a breakthrough infection. But not everyone is at the same risk. Four things appear to contribute to how well you are protected by vaccination.

1. Vaccine type

Clinical trials found that the Moderna vaccine reduced a person’s risk of developing symptomatic COVID-19 by 94%, while the Pfizer vaccine reduced this risk by 95%. The Johnson & Johnson and AstraZeneca vaccines performed less well, reducing this risk by about 66% and 70% respectively (though protection offered by the AstraZeneca vaccine appeared to rise to 81% if a longer gap was left between doses).

2. Time since vaccination

The length of time since vaccination is one of the reasons why the debate over booster immunizations is growing in intensity.

Early research, still in preprint (not yet peer reviewed), suggests that the Pfizer vaccine’s protection wanes over the six months following vaccination. Another preprint from Israel also suggests that this is the case. It’s too soon to know what happens to vaccine efficacy beyond six months in the double vaccinated, but it’s likely to reduce further.

3. Variants

Another important factor is the variant of the virus that you’re facing. The reductions in risk above were calculated largely by testing vaccines against the original form of the coronavirus.

But when facing the Alpha variant, data from Public Health England suggests that two doses of the Pfizer vaccine is slightly less protective, reducing the risk of getting COVID-19 symptoms by 93%. Against Delta, the level of protection falls even further, to 88%. The AstraZeneca vaccine is also affected this way.

The COVID Symptom Study backs all of this up. Its data suggests in the two to four weeks after receiving your second Pfizer jab, you’re around 87% less likely to get COVID-19 symptoms when facing delta. After four to five months, that figure falls to 77%.

4. Your immune system

It’s important to remember that the above figures refer to average risk reduction across a population. Your own risk will depend on your own levels of immunity and other person-specific factors (such as how exposed you are to the virus, which might be determined by your job).

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