Individuals with compromised immune systems, whether because of immunodeficiency or immuno suppressive therapy, are among those most susceptible to COVID19. In fact, people who are immunocompromised are doubly susceptible. On the one hand, people who are immuno compromised are more likely to suffer the gravest consequences of SARS CoV2 infection, including severe or fatal disease. 1, 2 On the other hand, such individuals are less likely to mount a sufficient immune response to COVID19 vaccination. Although the evidence base is skewed towards mRNA vaccines, the reduced vaccine response in people who are immunocompromised compared with people who are not immunocompromised appears to be a general phenomenon across COVID19 vaccines and vaccine platforms. In a systematic review, seroconversion rates after two COVID19 vaccine doses (pooling across all studies and platforms) were 99%(95% CI 98–100) for people who are not immunocompromised, 92%(88–94%) for patients with solid cancer, 78%(69–95) for patients with immunemediated inflammatory disorders, 64%(50–76) for patients with haematological cancer, and 27%(16–42) for recipients of transplants. 3 Corroborating these findings, people who are immuno compromised exhibit reduced protection against symptomatic and severe COVID19 on the basis of realworld effectiveness data (available for mRNA and vectored vaccines), 4–6 and make up greater than 40% of hospitalised breakthrough cases despite representing a much smaller proportion of the general population. 7

To mitigate the risk of COVID19 among vaccinated people who are immunocompromised, countries have increasingly opted to offer these individuals an additional vaccine dose. As of Oct, 2021, this policy has been recommended by WHO for people who are moderately or severely immunocompromised. 8 Notably, such policies should be considered separately from those relating to booster doses—given once an initially sufficient immune response rate in a vaccinated population has waned over time. 9 Rather, offering additional doses to people who are immunocompromised should be considered part of an extended primary series that seeks to increase the proportion of individuals in the population who achieve a sufficient protective immune response to begin with.