Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30

Many groups worldwide are trying to develop vaccines that protect against a wide range of coronaviruses and prevent (26) _________ pandemic. These efforts have now been boosted by the discovery that some healthcare workers had pre-existing immunity to the SARS-CoV-2 virus during the first wave of the pandemic. During the first half of 2020, around 700 healthcare workers in the UK were tested weekly as part of a crowdfunded study called Covid Sortium. Most of these people, who wore protective equipment, never tested positive for covid-19 in PCR tests or developed Covid-19 antibodies - proteins that (27) _________ to the outside of viruses, preventing cells from being infected. (28) __________, when Leo Swadling and Mala Maini at University College London and their colleagues looked more closely, they found some of those (29) ____________ tested negative had a protein in their blood that is linked to covid-19 infection, as well as T cell responses to the SARS-CoV-2 virus. T cells are (30) _________ of the immune system.

Adapted from https ://www. new scientist, com/