LONDON—The first study to directly compare immune reactions between Pfizer’s and AstraZeneca’s COVID-19 vaccines found strong and broadly similar antibody responses in over-80-year-olds after a first dose of either shot, scientists said on Wednesday. The UK study also found that a critical component of the immune system known as T cells showed a more enhanced response in those who got the AstraZeneca/Oxford University vaccine than in those who got the Pfizer/BioNTech one. The scientists behind the study, which analysed blood samples from 165 people in Britain aged between 80 and 99 who had been given a first COVID-19 vaccine dose, said that finding merited further investigation. While antibodies can block the coronavirus’s ability to enter human cells, T cells can act as broader protection by attacking and killing any cells that have been infected with the virus. “These vaccines are both equivalent and effective at inducing antibody responses in the …