The relationship between Covid-19 infections and antibodies: What do the data show?

Antibody testing

Covid-19 infections have recently reached their highest levels across many parts of the UK. At the same time, our data show that antibody levels are high. Sarah Crofts explains more about how this is possible.

Figures from the COVID-19 Infection Survey estimate that nearly 5 million people living in the UK were infected with Covid-19 in the last week of March 2022. Figures also show that 99% of the population are estimated to have antibodies to the virus. These will be from vaccination, a previous infection, or both. Antibodies provide immune protection against the virus, so why are so many people infected?

In answering this, it is important to know that immune systems vary and different people will have differing amounts of antibodies, which can be measured from a blood sample.

COVID-19 variants

We have seen many variants of COVID-19 since the pandemic began and some variants of the virus require more antibodies to prevent infection compared to others. It can be thought of in the same way as how people in a family are all a bit different. The different variants of COVID-19 vary, which means they are recognised differently by antibodies in the immune system. Although Omicron is closely related to earlier variants, it is different enough that we need more antibodies to protect us to the same degree.

This means that antibodies gained from either previous infection or vaccination aren’t always enough to stop someone becoming infected and testing positive with Omicron. However, antibodies continue to provide important protection as they give us an advantage in fighting the virus.


The impact of vaccines

Analysis has shown that people who are vaccinated or have had a previous infection are less likely to be infected (test positive), even with Omicron. Of those who do become infected, they are much less likely to suffer serious illness. While the infection rate is now more than three times higher than it was in January 2021, the number of people requiring mechanical ventilators is much less – for every 10 people on a ventilator then, only 1 ventilator is needed now.

When the COVID-19 Infection Survey first started to measure antibodies, before vaccines were rolled out, a person would ‘test positive’ for antibodies if they had greater than 42 nanograms of antibodies per millilitre of blood (ng/ml). This is the level that the test was CE marked against as providing a greater than 99% sensitivity and specificity to detect whether someone had had COVID before. Below this level, their result was classified as ‘negative’. However, as the vaccination programmes rolled out, many people in the population rapidly developed antibodies above this level.


Improved thresholds

More recently, we have been using a threshold of 179 ng/ml, based on research by our academic partners, suggesting this level provides good protection against infection with the Delta variant. We are still collecting data to work out the equivalent threshold for the same level of protection against the Omicron variants, but the high levels of infections we are currently seeing suggest the threshold will be higher.

Identifying the most helpful antibody threshold against which to measure protection requires continued monitoring and updating as new variants evolve. Classifying people into groups of ‘has antibodies’ and ‘does not have antibodies’ doesn’t tell us what antibody levels are needed to prevent infection or serious illness with different variants.

This is why it’s important we keep the thresholds we use under close review. We are continuing to explore ways in which to best represent the protection provided by antibodies in the population, and how these are changing – either increasing (with infection or vaccination) or decreasing (with antibody waning over time).


Sarah Crofts is Head of Analytical Outputs for the COVID-19 Infection Survey