A US nurse on Tuesday tested positive for Covid-19, eight days after having received a Pfizer vaccine shot. According to a report, his arm was sore for a day but he suffered no other side effects, after receiving the vaccine. However, six days later, the 45-year-old became sick while working a shift in the Covid-19 unit. Pfizer developed the vaccine along with German company BioNTech. The mRNA vaccine candidate, administered in two doses, has been found to be 95 per cent effective in preventing the disease.