To tackle the covid-19 pandemic, we need the most effective vaccines we can get. But even the best vaccines don’t work in everyone. How do you know if yours has worked?
All of the vaccines in use against the coronavirus can cause side effects, including a sore arm, fever, chills, headache and nausea, usually in the first two days after a jab.
These are more common after a second dose, and in people who have already been naturally infected with the coronavirus, according to on nearly 36,000 people in the UK who had the Pfizer/BioNTech vaccine.
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While side effects show your immune system is reacting to the virus, the absence of such signs doesn’t mean the jab has failed to work. Even with the second dose, only half of people in the UK study had a sore arm and one in five had a broader effect like fever. “People should not be worried if they don’t have a reaction,” says Deborah Dunn-Walters, chair of the British Society for Immunology’s covid-19 task force.
No matter what, it is crucial not to behave as if you are immune to the virus after a vaccine, says Paul Morgan at Cardiff University in the UK. It takes two to three weeks for a vaccine to start taking effect. Even after three weeks, vaccines won’t stop all infections, only reduce their severity and number in the population.
It still isn’t clear why some people catch the coronavirus after being vaccinated (see “No vaccine response”). But there is a way to know if a vaccine has had an effect on your immune system.
Some antibody tests that are used to detect natural coronavirus infections can also be used to detect antibodies made in response to vaccines three weeks after a shot.
Most tests look for antibodies that recognise the virus’s outer spike protein, which the virus uses to latch on to cells in the body, so they can identify people who have had a natural infection or a vaccine. Indeed, they can’t distinguish between them. But some identify antibodies recognising a molecule called the nucleocapsid protein, which isn’t contained in the vaccines, so wouldn’t detect the immune response in vaccine recipients.
And no test is perfect. Antibody tests have up to a 10 per cent rate of false negatives, telling someone they have no antibodies to the virus when they do, of using such tests in people two to four weeks after a proven infection. The false positive rate is lower, at around 2 per cent.
Current commercial tests give only a yes/no answer – they don’t quantify antibody levels, which tend to wane after a natural covid-19 infection. Nor do they give any indication of how powerful antibodies are against the different coronavirus variants.
Antibodies don’t tell the whole story about immunity. We have other parts of our immune system, including memory B-cells – the cells that make antibodies, but can’t be detected by an antibody test – and T-cells, which kill virus-infected cells directly.
Tests for T-cells are in development by companies such as UK-based Indoor Biotechnologies and German firm Qiagen. They could shed light on the body’s long-term response and help us know how often people will need covid-19 booster shots, says Maria Oliver at Indoor Biotechnologies.
At the moment, any tests, whether for antibodies or T-cells, are being used either for research or personal interest, not as proof of vaccination. Countries such as Sweden and Denmark are developing digital vaccination passports their residents could use to prove they have had a covid-19 vaccine prior to travelling, but this would involve a , not blood tests.
Rapid finger-prick tests for antibodies against the coronavirus could in future be used at places like airports. They wouldn’t prove someone is immune to the virus, but would show they have had the vaccine or a past infection.
Such antibody tests could have a role “in the fullness of time, if we get out of this critical phase when there’s so much virus around”, says Morgan. “But at the moment it’s more important to treat everyone who’s immunised as susceptible, and a vector of transmission to others.”
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No vaccine response
Even the best vaccines leave 5 per cent of vaccinated people susceptible; for some vaccines, that figure is more like 30 per cent. But it is unclear why.
Previous research on diseases such as influenza suggests many possible factors. Age, sex, nutritional status, gut microbes and teh state of the immune system may all play a role. In the case of covid-19, we know very little, says John Tsang at the US Center for Human Immunology in Maryland. “It’s a complicated issue.”
Unexpectedly, two factors that usually reduce vaccine efficacy – being older and being male – don’t appear to be at work this time, he says.
But baseline immune status probably matters. People with chronic inflammation generally respond worse to vaccines. Obesity can also be a factor, as it causes chronic inflammation, says Tsang.
Notably, clinical trials in lower-income countries, where volunteers may be exposed to higher pathogen and parasite loads, have lower vaccine efficacy rates.
Recent exposure to common-cold-causing coronaviruses could also influence the response. A covid-19 vaccine may simply re-activate the immune response to the cold rather than setting up a new one, reduce effectiveness.
