However, given the spread of Delta, there are concerns over how well the Johnson & Johnson vaccine holds up. While there is no systematic data collection on vaccine breakthrough cases nationally, reports from multiple locations suggest a higher breakthrough rate for Johnson & Johnson versus the other vaccines. In the Provincetown outbreak, a higher proportion of cases occurred from Johnson & Johnson recipients than from Pfizer or Moderna recipients, when adjusting for the number of vaccines given. Among people who are immune compromised, early studies suggest that there may be notable differences between the vaccines. One recent study, which has not been peer-reviewed, found that 33 percent of Johnson & Johnson-vaccinated dialysis patients did not develop antibodies, compared with 4 percent for Pfizer and 2 percent for Moderna…

Experience also suggests that getting an mRNA dose after the Johnson & Johnson or AstraZeneca vaccine is safe. According to C.D.C. data, more than 90,000 Johnson & Johnson recipients have already received an additional Covid-19 vaccine dose, even though this is not yet recommended. The mixing and matching of vaccines has already been endorsed in multiple European countries. Several prominent people have shared that they have gotten an mRNA dose after their Johnson & Johnson or AstraZeneca vaccine, including Chancellor Angela Merkel of Germany, multiple infectious disease experts and various pharmaceutical company executives. So far, none of them have reported serious adverse events, nor have there been unanticipated side effects in countries using the strategy.