What did we want to find out?
We wanted to find out which studies on the most commonly used COVID-19 vaccines in vulnerable subgroups have been published, and which outcomes were reported (e.g. effectiveness outcomes, safety, or immune response), to decide on the most relevant questions and answer these in further effectiveness systematic reviews (syntheses of the medical literature).
What did we do?
We searched medical databases and trial registries for studies on COVID-19 vaccines that were authorised for use in the European Union (European Medicines Agency (EMA)-approved) and those approved in at least 10 countries worldwide at the time of our search.
We included studies on additional conditions (comorbidities) that can reduce the immune reaction to vaccination, if they had more than 100 participants; they could include any age, sex, ethnicity, or country of recruitment.
We excluded studies looking at the general population and other than preselected COVID-19 vaccines and subgroups.
Once we found the studies, we categorised the vaccines into the following groups: EMA-approved COVID-19 vaccines, other COVID-19 vaccines, and schemes with different COVID-19 vaccines. We summarised the results in an interactive online map. We mapped the study outcomes, the country in which the study was conducted, the study design, and the vulnerable population.
What did we find?
We included 318 studies. Most studies came from high-income countries and included adults. We found that haematological malignancies (cancers that affect the blood, bone marrow, and lymph nodes) and solid tumours were examined in many studies, followed by people receiving dialysis and kidney transplants, rheumatic diseases, and others. Thirty-one studies included pregnant or breastfeeding women. The majority of studies explored mRNA vaccines (N = 283 and N = 153 for BNT162b2 and mRNA-1273) at two doses, and EMA-approved vaccines were more commonly administered than other vaccines and schemes with different COVID-19 vaccines.
Outcomes related to immunogenicity (how well a vaccine works, or the ability to stimulate the development of antibodies), especially the presence or absence of antibodies in the blood of patients or an estimate for the amount of these antibodies, were the most frequently reported outcome in more than 170 studies each. In addition, adverse events were assessed often (N = 126 studies), whilst SARS-CoV-2 infection was reported in only 80 studies.
What are the limitations of the evidence?
Due to the quick development of the pandemic, the research landscape may have changed. The newer Omicron variant has become the dominant variant, and a new vaccine has been approved by the EMA, which is not covered by our search.
How up-to-date is this evidence?
The evidence is up-to-date to December 2021.
What are the next steps?
Based on the overview from this review, we have decided to conduct two detailed systematic reviews on haematological malignancies and kidney transplant recipients.
Up to 6 December 2021, the majority of studies examined data on mRNA vaccines administered as standard vaccination schemes (two doses approximately four to eight weeks apart) that report on immunogenicity parameters or adverse events. Clinical outcomes were less commonly reported, and if so, were often reported as a secondary outcome observed in seroconversion or immunoglobulin titre studies. As informed by this scoping review, two effectiveness reviews (on haematological malignancies and kidney transplant recipients) are currently being conducted.
High efficacy in terms of protection from severe COVID-19 has been demonstrated for several SARS-CoV-2 vaccines. However, patients with compromised immune status develop a weaker and less stable immune response to vaccination. Strong immune response may not always translate into clinical benefit, therefore it is important to synthesise evidence on modified schemes and types of vaccination in these population subgroups for guiding health decisions. As the literature on COVID-19 vaccines continues to expand, we aimed to scope the literature on multiple subgroups to subsequently decide on the most relevant research questions to be answered by systematic reviews.
To provide an overview of the availability of existing literature on immune response and long-term clinical outcomes after COVID-19 vaccination, and to map this evidence according to the examined populations, specific vaccines, immunity parameters, and their way of determining relevant long-term outcomes and the availability of mapping between immune reactivity and relevant outcomes.
We searched the Cochrane COVID-19 Study Register, the Web of Science Core Collection, and the World Health Organization COVID-19 Global literature on coronavirus disease on 6 December 2021.
We included studies that published results on immunity outcomes after vaccination with BNT162b2, mRNA-1273, AZD1222, Ad26.COV2.S, Sputnik V or Sputnik Light, BBIBP-CorV, or CoronaVac on predefined vulnerable subgroups such as people with malignancies, transplant recipients, people undergoing renal replacement therapy, and people with immune disorders, as well as pregnant and breastfeeding women, and children. We included studies if they had at least 100 participants (not considering healthy control groups); we excluded case studies and case series.
We extracted data independently and in duplicate onto an online data extraction form. Data were represented as tables and as online maps to show the frequency of studies for each item. We mapped the data according to study design, country of participant origin, patient comorbidity subgroup, intervention, outcome domains (clinical, safety, immunogenicity), and outcomes.
Out of 25,452 identified records, 318 studies with a total of more than 5 million participants met our eligibility criteria and were included in the review. Participants were recruited mainly from high-income countries between January 2020 and 31 October 2021 (282/318); the majority of studies included adult participants (297/318).
Haematological malignancies were the most commonly examined comorbidity group (N = 54), followed by solid tumours (N = 47), dialysis (N = 48), kidney transplant (N = 43), and rheumatic diseases (N = 28, 17, and 15 for mixed diseases, multiple sclerosis, and inflammatory bowel disease, respectively). Thirty-one studies included pregnant or breastfeeding women.
The most commonly administered vaccine was BNT162b2 (N = 283), followed by mRNA-1273 (N = 153), AZD1222 (N = 66), Ad26.COV2.S (N = 42), BBIBP-CorV (N = 15), CoronaVac (N = 14), and Sputnik V (N = 5; no studies were identified for Sputnik Light). Most studies reported outcomes after regular vaccination scheme.
The majority of studies focused on immunogenicity outcomes, especially seroconversion based on binding antibody measurements and immunoglobulin G (IgG) titres (N = 179 and 175, respectively). Adverse events and serious adverse events were reported in 126 and 54 studies, whilst SARS-CoV-2 infection irrespective of severity was reported in 80 studies. Mortality due to SARS-CoV-2 infection was reported in 36 studies.
Please refer to our evidence gap maps for more detailed information.