Immune markers that indicate good vaccination protection

Seventeen months after SARS-CoV-2 was identified, 6 vaccines against COVID-19 were already available. These vaccines give effective protection against serious illness and death. They reduce asymptomatic infection and limit the spread of the virus. However, supply of vaccines is still limited despite huge production efforts. It is therefore important that other vaccines be made available. But as countries progress with their vaccination campaigns, it becomes more difficult to carry out large-scale clinical studies on new vaccines. British researchers at Oxford University and within the AstraZeneca pharmaceutical group have tried to identify markers in the immune system indicating good vaccinal protection against COVID-19 where large-scale trials are no longer possible.

The researchers analysed the protection given by the AstraZeneca vaccine. It appears to have 67% effectiveness against symptomatic infection, and 27% effectiveness against asymptomatic infection. They made correlations 28 days after the second injection of the vaccine between different immune markers and symptomatic or asymptomatic infections.

The researchers compared 171 vaccinated and infected people with 1 404 vaccinated non-infected people. They firstly measured levels of neutralising antibodies directed against SARS-CoV-2. Then they carried out virus neutralisation tests with the serum of vaccinated individuals. There was a clear correlation between high levels of immune system markers with good protection against symptomatic infection. However, no correlation was found between levels of these markers and the risk of asymptomatic infection.

However, this study did have certain limits: the authors did not compare different age groups since the sample studied was too small, though immunity does vary with age. In addition, the immune markers highlighted in this study may change according to vaccine technology. So an mRNA vaccine may not have the same markers of effectiveness.

In conclusion, it is clearly useful to have a good understanding of the immune system and its markers that indicate adequate vaccine protection. When numbers in a clinical study are small, these immune markers can help assess the efficacy of a candidate vaccine.