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US CDC recommends third dose of Moderna or Pfizer COVID-19 vaccines for organ transplant recipients

Keith Alcorn
16 August 2021
Image by Marco Verch. Creative Commons licence.

People who have undergone a solid organ transplant and who received the Pfizer or Moderna COVID-19 vaccines should receive a third dose, the US Centers for Disease Control and Prevention (CDC) recommended last week.

The recommendation also applies to people who are receiving treatment for liver cancer and to people with HIV with low CD4 counts.

The decision follows approval of third doses of the Pfizer and Moderna vaccines by the US Food and Drug administration. French and British health authorities have already issued recommendations for third booster doses for immunocompromised people and transplant recipients respectively.

The recommendation does not apply to recipients of the Johnson & Johnson single-dose vaccine. CDC said that there are not sufficient data to make a recommendation yet.

Immunocompromised people have weaker responses to vaccination with many types of vaccine.

Immunocompromised people are those who have:

  • been receiving active cancer treatment for tumors or cancers of the blood
  • received an organ transplant and are taking medicine to suppress the immune system
  • received a stem cell transplant within the last 2 years or are taking medicine to suppress the immune system
  • moderate or severe primary immunodeficiency (such as DiGeorge syndrome, Wiskott-Aldrich syndrome)
  • advanced or untreated HIV infection
  • active treatment with high-dose corticosteroids or other drugs that may suppress your immune response.

In its recommendations, US CDC drew attention to two studies showing that a high proportion of those hospitalised with COVID-19 after being fully vaccinated are immunocompromised.

A US study of COVID-19 hospital admissions at 18 academic medical centres between March and May 2021 identified 45 cases of COVID-19 in fully vaccinated people; 20 (44% of the total) cases occurred in people who were immunocompromised. Seven of these cases occurred in people who had undergone a solid organ transplant. By matching COVID-19 cases to hospital admissions for respiratory complaints or other disorders in people who tested negative for SARS-CoV-2, the investigators estimated that the effectiveness of the Pfizer and Moderna vaccines against severe illness was 86.9% but was reduced to 51.9% in people with a solid organ malignancy or transplant.

An Israeli study of COVID-19 hospital admissions at 17 hospitals in people fully vaccinated with the Pfizer vaccine identified 152 cases, 60 in immunocompromised people. Sixteen of these cases occurred in solid organ transplant recipients; only three cases in transplant recipients were classified as severe.

Public Health England has also reported on COVID-19 hospitalisations in around 39,000 people who have undergone transplants in England. Between December 2020 and June 2021, 76 people contracted COVID-19 two weeks or more after second vaccine dose. Of these 8% (6) died within 28 days of a positive COVID-19 test

US CDC says that a booster dose should be given at least four weeks after the second dose of the Pfizer or Moderna vaccines.

However, a small case series of 30 transplant recipients with negative or weak responses to the Moderna or Pfizer vaccines showed that two-thirds of those with negative antibody levels prior to a third booster dose (24 of 30) still had negative antibody levels two weeks after the booster dose. Those with weak antibody levels prior to boosting (6/30) achieved high antibody levels after boosting. In this study, 15 of the 30 patients received a booster dose with a different type of vaccine, the Johnson & Johnson adenovirus-vectored vaccine.

However, a larger study in 101 solid-organ transplant recipients (including 12 liver transplant patients) who received a third booster dose of the Pfizer or Moderna vaccine found that 26 out of 59 people (44%) who had negative antibody responses prior to the booster developed antibody responses after the third dose.

Neither study looked at cellular immune responses, which play a key role in preventing severe illness. Although cellular immune responses are reduced in immunocompromised people, different causes of immunosuppression may have different effects on cellular immunity. More research is needed to determine whether cellular immune responses to SARS-CoV-2 vaccination is less strong in transplant recipients and how cellular immune responses to vaccination affect the subsequent risk of infection or illness.


US Centers for Disease Control. COVID-19 vaccines for moderately to severely immunocompromised people. 13 August 2021.

Tenforde MW et al.Effectiveness of SARS-Cov-2 mRNA vaccines for preventing breakthrough COVID-19 hospitalizations in the United States. Bio Rxiv pre-print, 8 July 2021

Brosh-Nissimov T et al. BNT162b2 vaccine breakthrough: clinical characteristics of 152 fully vaccinated hospitalized COVID-19 patients in Israel. Clinical Microbiology and Infection,

Werbel W. Safety and immunogenicity of a third dose of SARS-CoV-2 vaccine in solid organ transplant recipients: a case series. Annals of Internal Medicine, published online, 15 June 2021.

Kamar N et al. Three doses of an mRNA Covid-19 vaccine in solid-organ transplant recipients. New England Journal of Medicine, 385:661-662, 2021.

Full image credit: A man is getting an injection with a syringe at hospital. Image by Marco Verch Professional Photographer. Available at under a Creative Commons licence CC BY 2.0.