An ongoing phase three clinical trial of Pfizer-Biontech's coronavirus vaccine is confirming the protection lasts at least six months after the second dose. An ongoing phase three clinical trial of Pfizer-Biontech's coronavirus vaccine is confirming the protection lasts at least six months after the second dose. âThis is great news that puts into numbers what we've been expecting. The Pfizer from the beginning and both MRNA vaccines, messenger RNA vaccines. So, Pfizer and Moderna have had really very robust responses in individuals who got the vaccine,â said Infectious Disease Specialist with Central Michigan University College of Medicine Nicholas Haddad. The vaccine remains more than 91% effective against COVID-19 for six months. The vaccine appears to be fully effective against the South Africa variant, the dominant virus strain circulating, and the one researcher's fear has evolved to evade the vaccines.