To tackle the covid-19 pandemic, we need the most effective vaccines we can get. But even the聽best vaccines don鈥檛 work in everyone. How do you know if yours has worked?
All of the vaccines in use against聽the coronavirus can cause聽side effects, including a sore arm, fever,聽chills, headache and nausea, usually in the first two days after a聽jab.
These are more common after a聽second dose, and in people who聽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鈥檛 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. 鈥淧eople should not be worried if they don鈥檛 have a reaction,鈥� says Deborah Dunn-Walters, chair of the British Society for Immunology鈥檚 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鈥檛 stop all infections, only reduce their severity and number in the population.
It still isn鈥檛 clear why some people catch the coronavirus after being vaccinated (see 鈥�No vaccine response鈥�). 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鈥檚 outer spike protein, which the virus uses聽to latch on to cells in the body,聽so they can identify people聽who have had a natural infection or a聽vaccine. Indeed, they can鈥檛 distinguish between them. But聽some identify antibodies recognising a molecule called the聽nucleocapsid protein, which isn鈥檛 contained in the vaccines, so聽wouldn鈥檛 detect the immune response in vaccine recipients.
And no test is perfect. Antibody tests have up to a 10 per cent rate聽of聽false negatives, telling someone聽they have no antibodies to the virus when they do, of using such聽tests in people two to four weeks after a proven infection. The聽false positive rate is lower, at聽around 2聽per cent.
Current commercial tests give only a yes/no answer聽鈥� they don鈥檛 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鈥檛 tell the whole story about immunity. We have other parts of our immune system, including memory B-cells聽鈥� the cells that make antibodies, but can鈥檛 be detected by an antibody test 鈥� and T-cells,聽which kill virus-infected cells directly.
Tests for T-cells are in development by companies such聽as UK-based Indoor Biotechnologies and German firm Qiagen. They could shed light on the body鈥檚 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聽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鈥檛 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聽role 鈥渋n the fullness of time, if we get out of this critical phase when there鈥檚 so much virus around鈥�, says Morgan. 鈥淏ut at the moment it鈥檚 more important to treat everyone who鈥檚 immunised as聽susceptible, and a vector of聽transmission to others.鈥�
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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. 鈥淚t鈥檚 a complicated issue.鈥�
Unexpectedly, two factors that usually reduce vaccine efficacy 鈥� being older and being male 鈥� don鈥檛 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.
