Four factors that increase the risk of vaccinees contracting COVID

Two weeks after your second dose of COVID-19 vaccine, the protective effects of the vaccination will be at their peak. At this point you are fully immunized. If you still contract COVID-19 after this point, you have experienced a “breakthrough” infection. Generally speaking, breakthrough infections are similar to regular COVID-19 infections in unvaccinated people – but there are a few differences. Here’s what to watch out for if you’ve had both jabs.

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

However, the other two most common symptoms in unvaccinated people are fever and a persistent cough. These two “classic” symptoms of COVID-19 become much less common after you have had your injections. One study found that people with breakthrough infections are 58% less likely to have a fever than unvaccinated people. On the contrary, COVID-19 after vaccination has been described as a common cold for many.

Vaccinated people are also less likely than unvaccinated people to be hospitalized if they develop COVID-19. They are also likely to have fewer symptoms during the early stages of the illness and are less likely to develop long-term COVID.

The reasons why the disease is milder in people who are vaccinated could be because vaccines, if they don’t block the infection, seem to cause infected people to have fewer virus particles in their bodies. However, this remains to be confirmed.

What increases the risk?

In the UK, research has found that 0.2% of the population – or one in 500 people – gets a breakthrough infection when fully vaccinated. But not everyone is at the same risk. Four things seem to contribute to the quality of your immunization protection.

1. Type of vaccine

The first is the specific type of vaccine you received and the relative risk reduction offered by each type. Relative risk reduction is a measure of the extent to which a vaccine reduces a person’s risk of developing COVID-19 compared to a person who has not been vaccinated.

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

2. Time since vaccination

But these numbers don’t paint the full picture. It is becoming increasingly clear that the time elapsed since vaccination is also important and this is one of the reasons why the debate over booster vaccinations is intensifying.

Early research, still in prepublication (and therefore yet to be reviewed by other scientists), suggests that the protection of the Pfizer vaccine wanes over the six months following vaccination. Another preprint from Israel also suggests this is the case. It is too early to know what happens to the efficacy of the vaccine beyond six months in double vaccines, but it is likely to decline further.

3. Variants

Another important factor is the variant of the virus you are facing. The above risk reductions were calculated in large part by testing vaccines against the original form of the coronavirus.

But in the face of the alpha variant, data from Public Health England suggests that two doses of the Pfizer vaccine are slightly less protective, reducing the risk of developing symptoms of COVID-19 by 93%. Against Delta, the level of protection drops even more, to 88%. The AstraZeneca vaccine is also affected in this way.

The COVID Symptom Study supports all of this. Its data suggests that within two to four weeks of receiving your second Pfizer jab, you’re about 87% less likely to have COVID-19 symptoms when facing the delta. After four to five months, that number drops to 77%.

4. Your immune system

It is important to remember that the numbers above refer to the average reduction in risk in a population. Your own risk will depend on your own level of immunity and other person-specific factors (such as your level of exposure to the virus, which may be determined by your job).

Immune capacity generally decreases with age. Long-term medical conditions can also affect our response to vaccination. The elderly or people with weakened immune systems may therefore have lower levels of protection from the COVID-19 vaccine, or may have their protection waned more quickly.

It should also be remembered that the most clinically vulnerable received their vaccines first, perhaps more than eight months ago, which may increase their risk of experiencing a breakthrough infection due to waning protection.

Do you need to worry?

Vaccines further significantly reduce your chances of contracting COVID-19. They protect even more against hospitalization and death.

However, it is worrying to see breakthrough infections, and the concern is that they could increase if vaccine protection, as suspected, wanes over time. Therefore, the UK government is planning to give a booster dose to the most vulnerable people and is also considering whether the boosters should be given more widely. Other countries, including France and Germany, are already planning to offer reminders to groups considered to be at higher risk of COVID-19.

But even boosters end up being used, it shouldn’t be interpreted as a vaccine that doesn’t work. And in the meantime, it is essential to promote vaccination to all eligible people who have not yet been vaccinated.