Date of Update: 5 January 2021
So far, 10 different platforms have been used in the development of these vaccines: 1) protein subunit (PS); 2) inactivated virus (IV); 3) non-replicating viral vector (VVnr); 4) RNA; 5) DNA; 6) virus-like particle (VLP); 7) replicating viral vector (VVr); 8) live attenuated virus (LAV); 9) VVnr + antigen-presenting cell (APC); and 10) VVr + APC. The first 5 platforms represent 84% of the vaccine candidates in clinical phase of development (1). Currently, 14 developers have COVID-19 vaccines in phase 3 studies, as shown in Table 1.
Among the vaccines in phase 3 studies, 6 have been authorized in several countries for emergency use (Pfizer/ BioNTech, Moderna, Gamaleya, Sinovac, Sinopharm, AstraZeneca/Oxford) and have started to be administered to health professionals and the elderly in Europe, the Americas, the Eastern Mediterranean region and Singapore. In the healthy population, projected clinical efficacy based on phase 2 and phase 3 studies varies from more than 50% to 95% (2-5).
Despite the achievements in such a short time, many questions remain unanswered, such as the titers of neutralizing antibodies for a COVID-19 vaccine to protect humans, the duration of vaccine-induced immunity, the possibility of occurrence of antibody-dependent enhancement (ADE) phenomenon, antibody responses in transplant recipients, among others. These and other queries that may arise with the expanded use of the COVID-19 vaccines will likely be clarified over time.
Table 1. Summary of phase 3 COVID-19 vaccines’ information.
Developers |
Vaccine platform/type |
No. of doses |
Schedule days |
Route |
Sinovac |
Inactivated |
2 |
0, 14 |
IM |
Wuhan Institute of Biological Products/ Sinopharm |
Inactivated |
2 |
0, 21 |
IM |
Beijing Institute of Biological Products/ Sinopharm |
Inactivated |
2 |
0, 21 |
IM |
Bharat Biotech |
Inactivated |
2 |
0, 28 |
IM |
Institute of Medical Biology and Chinese Academy of Medical Sciences |
Inactivated |
2 |
0, 28 |
IM |
University of Oxford/ AstraZeneca |
VVnr/ChAdOx1-S |
2 |
0, 28 |
IM |
CanSino Biological Inc/Beijing Institute of Biotechnology |
VVnr/Ad type 5 |
1 |
- |
IM |
Gamaleya Research Institute |
VVnr/rAd26-S + rAd5-S |
2 |
0, 21 |
IM |
Janssen Pharmaceutical Companies |
VVnr/Ad type 26 |
2 |
0, 56 |
IM |
Moderna/NIAID |
RNA/LNP encapsulated mRNA |
2 |
0, 28 |
IM |
BioNTech/Fosun Pharma/Pfizer |
RNA/LNP encapsulated mRNA |
2 |
0, 28 |
IM |
CureVac AG |
RNA/CVnCoV Vaccine |
2 |
0, 28 |
IM |
Novavax |
Protein subunit/SARS CoV-2 rS – Matrix M1 adjuvant |
2 |
0, 21 |
IM |
Anhui Zhifei Longcom Biopharmaceutica, Chinese Academy of Sciences |
Protein subunit/ recombinant SARS CoV-2 vaccine (CHO cells) |
3 |
0, 28, 56 |
IM |
While prioritization of vaccine is generally determined by the federal, state and local health authorities, transplant recipients should be included in groups for earlier vaccination due to the risk for severe COVID-19. Immunocompromised patients, including transplant recipients, have not been included in studies performed to date. As such, transplant recipients should be counseled that the effectiveness and safety profile of these vaccines for them are not currently known. As these are not live virus vaccines, it is unlikely that these vaccines would pose additional risks, Transplant recipients may have decreased vaccine responses compared to the general population, and thus should be advised regarding the importance of maintaining all current guidance to protect themselves even after vaccination, including continuing to use masks, focus on hand hygiene and social distancing. Additionally, caregivers and household contacts should be strongly encouraged to get vaccinated when available to them to protect the patient.
Inactivated vaccines, protein subunit recombinant or virus-like vaccines are considered safe to be administered to transplant populations. Particle vaccines have been used for decades in transplant vaccination programs (e.g., influenza, hepatitis B and HPV vaccines). RNA vaccines (BioNTech/Pfizer, Moderna) and non-replicating viral vector vaccines (Oxford/AstraZeneca, Gamaleya) are also considered safe vaccines, but have never been used in the transplant scenario. Vigilance will be necessary to determine if the induced protective immunity is not associated with an increased risk for rejection episodes or the development of graft versus host disease (GVHD).
Live attenuated vaccines are generally contraindicated in SOT recipients and may be used with restrictions in HSCT recipients. Replicating viral vector vaccines are not recommended in transplant populations at this moment.
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