London, Oct 6 : A person's immune response to variants of SARS-CoV-2, the virus that causes Covid-19, depends on their previous exposure -- and differences in the focus of immune responses will help scientists understand how to optimise vaccines in the future to provide broad protection, suggests a study. The study found that the variant of SARS-CoV-2 a person gets first exposed to determines how well the immune system responds to different parts of the virus, and how protected the person is against other variants. It also means that the same Covid-19 vaccine might work differently for different people, depending on which variants of SARS-CoV-2 they have previously been exposed to and where their immune response has focused. The discovery underlies the importance of continuing surveillance programmes to detect the emergence of new variants, and to understand differences in immunity to SARS-CoV-2 across the population. It will also be important for future vaccination strategies, which must consider both the virus variant a vaccine contains and how immune responses of the population may differ in their response to it. "It was a surprise how much of a difference we saw in the focus of immune responses of different people to SARS-CoV-2. Their immune responses appear to target different specific regions of the virus, depending on which variant their body had encountered first," said Samuel Wilks at the University of Cambridge's Center for Pathogen Evolution in the Department of Zoology. "Our results mean that if the virus mutates in a specific region, some people's immune system will not recognise the virus as well -- so it could make them ill, while others may still have good protection against it," he added. For the research, published in the journal Science, the team collected 207 serum samples -- extracted from blood samples -- from people who had either been infected naturally with one of the many previously circulating SARS-CoV-2 variants, or who had been vaccinated against SARS-CoV-2 with different numbers of doses of the Moderna vaccine. They then analysed the immunity these people had developed, and found significant differences between immune responses depending on which variant a person had been infected with first. The research used a technique called "antigenic cartography" to compare the similarity of different variants of the SARS-CoV-2 virus. This measures how well human antibodies, formed in response to infection with one virus, respond to infection with a variant of that virus. It shows whether the virus has changed enough to escape the human immune response and cause disease. The resulting "antigenic map" shows the relationship between a wide selection of SARS-CoV-2 variants that have previously circulated. Omicron variants are noticeably different from the others -- which helps to explain why many people still succumbed to infection with Omicron despite vaccination or previous infection with a different variant. /IANS