How Can You Tell If You're Immune After Having The Covid Jab?

HuffPost UK reader Sue asked whether there's a test for the level of immunity after having the coronavirus jab. Here's what experts say.

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HuffPost UK reader Sue asked: “Is there a test for the level of immunity after having the jab?”

There are a lot of things we know about Covid antibodies – but equally, many things we’re still in the dark about.

Antibodies are a type of blood protein produced when your body fights off a specific illness – so when you catch Covid-19, for instance, and your body manages to overcome it, you’ll produce antibodies that know how to fight the virus better next time so you hopefully don’t become as ill.

We know that in people who’ve been infected with the virus, antibodies last at least six months in their system. They might actually last longer, but because SARS-CoV-2 has only been knocking around for a relatively short amount of time, we can’t say for sure that people have protection longer than that.

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How long do vaccines provide immunity for?

Vaccines give protection for a minimum of three months – but this could end up lasting longer. Vaccines are generally believed to induce stronger and longer lasting immunity than natural immunity. Based on research from other vaccines, scientists believe the jab will provide immunity for well over a year.

A UK-wide study involving more than 400,000 people found Covid-19 infections fell significantly – by 65% – after a first dose of the AstraZeneca or Pfizer vaccines. The same study found two doses of the Pfizer vaccine offered similar levels of protection against the virus as previously having the virus. There wasn’t enough data to determine whether the same held true for the AstraZeneca jab.

Vaccination was just as effective in those aged 75 and over or with underlying health conditions, as it was in those under 75 or without health conditions. In less than 5% of people, there were low responses to both vaccines.

Can we find out our individual level of immunity after being jabbed?

There’s a lot of variation in our bodies’ responses to the jabs – and indeed to having Covid – so people are interested whether tests can tell us our level of immunity after having them. Sadly, the simple answer is: no. We don’t yet have widely available tests that can tell us specific things about our level of immunity.

Antibody tests – of which there are now many on the market – can give a relatively simple ‘yes’ or ‘no’ answer to whether we have Covid antibodies, and therefore some immunity to the virus.

There are two main types of antibody tests, according to the Covid Symptom Study app. The first, called Anti-N tests, looks for antibodies that recognise a molecule inside the virus called the nucleocapsid, which are only produced if you’ve previously caught the virus. The second – dubbed Anti-S tests – detect antibodies against the spike protein (hence the ‘S’) on the surface of the virus. As the vaccines are based on the spike protein, these tests can detect antibodies produced both through previous infection and vaccination.

The higher the concentration of antibodies in the blood, the better the protection, says Professor Charles Bangham, chair of immunology at Imperial College London.

But it’s worth noting the protection is rarely complete. This means a person with antibodies could still be reinfected with the same virus – as Professor Paul Hunter, from the University of East Anglia, explains: “Immunity is not a binary yes or no thing, but you can have some partial immunity.”

If you do have Covid antibodies that show up on tests – whether post-vaccine or illness – and you get reinfected, the illness will usually be milder. This is because the virus multiplies less, and so causes less disease and less risk of transmission to other people, says Prof Bangham.

How accurate are antibody tests?

If you decide to use an antibody test, there are a number of things to bear in mind. Dr Joshua Moon, research fellow in the Science Policy Research Unit at the University of Sussex Business School, says the ability of non-lab-based tests is variable. There’s also a high variation in how accurate tests are when used on blood taken from a finger, as opposed to drawn by a professional.

Another issue is the unknowns around how protective antibodies are, he says, because immunity doesn’t just involve antibodies but T cells and B cells, too – more on this later.

Why can’t tests tell us our exact immunity?

The reason why tests don’t exist to tell us the precise level of immunity we have from a vaccine or illness is because the immune system is just so complex.

Dr Alexander Edwards, associate professor in biomedical technology at the University of Reading, says immune responses vary massively from human to human, which makes things complicated when it comes to figuring out levels of protection.

The variation in our immune systems is actually a good thing, he says, because if we were all the same, “pathogens would be far more dangerous and better at escaping”. But as a result of this variation, it’s “incredibly difficult” to look at any one individual and predict how well they will be protected purely based on their response to vaccination, he says.

Some people test positive for Covid and, months later, antibody tests then show up as negative. So, does this mean they don’t have any immunity at all?

Well no, because antibodies are only one part of the very complicated immune system. “T lymphocytes [or T cells] are essential for a normal, efficient immune response to a virus, and for making good ‘immune memory’,” explains Professor Charles Bangham.

“Some people have good immunity to a virus with very low levels of antibodies, because they have a strong T lymphocyte response to the virus,” he continues. “But since T lymphocytes are trickier and more expensive to analyse, antibodies are more often used as an indicator of immunity.” There are T cell tests, but they’re rare – and also more complicated to do than antibody tests.

Ultimately, adds Prof Bangham, the only true measure of immunity is what happens when a person is reinfected with the virus. If you catch Covid-19 for a second time and your symptoms are mild – or you’re asymptomatic – it’s likely you’ve developed some level of immunity.

Experts are still learning about Covid-19. The information in this story is what was known or available at the time of publication, but guidance could change as scientists discover more about the virus. To keep up to date with health advice and cases in your area, visit gov.uk/coronavirus and nhs.uk.