Vaccination Immunity

How can we measure the immune response to coronavirus vaccines?

Dr Claire Merrifield MBBS MRCGP PhD, Dr Shohaib Ali BSc MBBS

May 4th, 2021

We’re starting to learn more about how long any immunity to coronavirus may last and how well vaccines may protect against future infection. One of the main measures we have of the immune response to coronavirus and to vaccination is antibody production

What are coronavirus antibodies?

Antibodies are molecules produced by the immune system to fight infections. When the body is infected with a new virus, the immune system produces antibodies against different parts of the virus. For coronavirus, we can measure several different types of antibody. One is to the body of the virus, called the nucleocapsid antibody (N-antibody). If you have this antibody present in your bloodstream it means that, at some point, you have been exposed to coronavirus (SARS-CoV-2) and your body has made defenses against it. The other type of antibody we can detect is the spike antibody (S-antibody) This will be present if you have been infected with coronavirus but also if you have been vaccinated as the vaccine tricks our body into producing an immune response against the spike protein. Often these antibodies stay in the body for a long time and provide protection against future infection.

Antibody response to infection

It takes around 5-14 days for your body to produce antibodies once exposed to coronavirus and there is ongoing research to see how long this antibody response lasts.

A study of 1,107 people in Iceland who recovered from coronavirus found that about 90% of people developed antibodies. Four months later there was no decline in their antibody levels1. Another study found that even people with mild cases of COVID-19 produce antibodies for around 5-7 months. Interestingly they found that the N-antibodies tended to decline quicker than the S-antibodies2.

This is important as it suggests you are protected from getting COVID for several months after the initial infection. However, re-infection with coronavirus does happen although it seems to be relatively rare and if it does happen, the infection tends to be less severe3.

Antibody response to vaccination

Vaccines work by stimulating the body to create an immunological ‘memory’ of an infection without having the illness itself. Part of this defense is the production of antibodies to attach to the virus and alert the rest of the immune system to attack. All the major coronavirus vaccines (including AstraZeneca/Oxford and Pfizer/BioNTech used in the UK) work by creating a response against the virus spike protein. This means that after two doses of vaccine most people who are protected will have spike antibodies in their bloodstream.

Checking antibody levels in people who have been vaccinated is one way of seeing how effective a vaccine has been. For example, with hepatitis B, we know that a blood level of over 100 IU/L is protective. However, we don’t yet have much data on how coronavirus spike antibody levels correlate with protection or even if spike antibodies are absolutely necessary for protection. We are still in the early days of vaccine rollout in the UK but the REACT study shows that in those people who had received 2 doses of the Pfizer vaccine, 91% had S-antibodies4. The key question at the moment is how long protection from the vaccine will last? We will have to wait some time for the answer to this question, but regular checking of antibody levels is likely to be part of understanding the immune response to vaccination.

What does my antibody test result mean?

N-antibody positive: You likely had infection with coronavirus at some point in the past.

S-antibody positive: You either had infection with coronavirus at some point in the past or you have had the vaccination.

N- and S-antibody positive: You were infected with coronavirus some point in the past and you may also have been vaccinated.

S-antibody positive and N-antibody negative: Most likely you’ve been vaccinated against the virus but have not had previous infection.

You can check your vaccination response with a spike antibody test here.