September/October 2021Vol. XXXIV No. 1

Booster Recommendations and the Delta Variant

James C. S. Liu, MD

There has been a great deal of confusion and concern around getting an additional dose of CoViD-19 vaccine. This concern has been driven by the Delta variant, now the dominant SARS-CoV-2 variant in the United States because it propagates much more prolifically and quickly than previous sub-groups of SARS-CoV-2 (for more information, visit the MIT Medical blog page about the Delta variant <>).

Vaccines help to fight off SARS-CoV-2, but it takes time for your body to recognize the presence of an invader, figure out which part of the immune response will help to neutralize it, and ramp up production. The Delta variant reproduces so rapidly that we believe some of the treatment “failures” of vaccines may simply represent an immune system that is overwhelmed by a fast-reproducing virus. For example, a pre-print study in Singapore <> suggests that people with vaccine breakthrough infections can clear the Delta variant much faster than people who have not been immunized. (An MIT Medical blog post <> does a deeper dive on breakthrough infections.)

A number of studies (reviewed in this presentation <> from the Centers for Disease Control or CDC) suggest that recipients of the Pfizer/BioNTech immunization were slightly likelier to have breakthrough infections more than six months after getting their second shot. A study in Israel <> suggests that a third shot is effective in preventing breakthrough infections from Delta in older people at least temporarily (we have no data on how long that protection lasts).

Another study <> followed Moderna vaccine recipients through the completion of the blinded phase (roughly the end of March, before Delta emerged). This study showed that breakthrough infections started to happen some three months after full immunization (once antibodies started to wane) but even seven months out, severe illness was not happening among this group, regardless of age or comorbidities. Data on the Janssen/Johnson and Johnson vaccine <> would suggest that protection rates stayed about the same or perhaps even increased somewhat.

All of the vaccines show high rates of efficacy in preventing severe illness, hospitalization, and death  <>. Multiple studies suggest that the Delta variant is worst among those unimmunized against CoViD-19.

The Food and Drug Administration (FDA), CDC, and Advisory Committee on Immunization Practices (ACIP, which makes the official recommendations on immunizations in the United States) have held meetings to review the data in the past two weeks. The weak strength of the data have led to disagreement among panel members about immunization policy recommendations. The consensus is that a third shot should be given to recipients of the Pfizer/BioNTech vaccine, at least six months after finishing their primary vaccine series, if they are 65 years and older (this was the group likeliest to show waning immunity), residents of long-term care settings 18 years and older (because of the high risk of transmission of infection), and people aged 50-64 years with medical conditions such as cancer, chronic kidney disease, chronic obstructive pulmonary disease (COPD), diabetes mellitus, congestive heart failure, coronary artery disease, cardiomyopathies, obesity, pregnancy, and recent pregnancy (because they are at increased risk of poor outcomes if they get CoViD-19).

FDA, CDC, and ACIP have suggested that a third shot may be given to some recipients of the Pfizer/BioNTech vaccine where the evidence of benefit is not as clear, including people aged 18-49 with the above medical conditions, and people aged 18-64 with occupational or institutional risks for exposure and transmission (first responders including healthcare workers, firefighters, police, and congregate care staff; education staff such as teachers, support staff, daycare workers; food and agriculture workers; manufacturing workers; corrections workers; U.S. Postal Service workers; public transit workers; grocery store workers; residents of correctional facilities and homeless shelters).

At the moment, there is no recommendation for additional immunization for recipients of the Moderna or Janssen/Johnson and Johnson vaccines, as the data we have suggest that their protection has not waned significantly. However, more data are being collected, and recommendations may change as the data are analyzed. Dr. Anthony Fauci commented <> that data are also being analyzed in patients who received several different types of CoViD-19 vaccines to see if one strategy may work better to prevent transmission and disease, and additional recommendations may be forthcoming on the idea of mixing and matching vaccines as well.

The CDC maintains and regularly updates pages with information about CoViD-19 and immunization. Check for updates on booster recommendations and for regularly updated science briefs with deep dives on the data on vaccines, transmission, and variants. 

MIT Medical maintains an excellent blog on CoViD-19 at with thoughtful articles on masks, traveling, immunizations, variants, and much more. MIT Medical’s recommendations will remain consistent with ACIP recommendations as they are updated.