Given the contradictory information and statements issued by individuals about the possible harms of the SARS-CoV-2 vaccine, which could contribute to an irrational attitude toward vaccination and the avoidance of this form of COVID-19 prevention, we feel the need to inform our members about several facts.

Rok Čivljak, President
Croatian Catholic Medical Society (HKLD)

According to the world news, vaccination against SARS-CoV-2, the causative agent of COVID-19, began in the United Kingdom on December 8, 2020, using the mRNA vaccine produced by Pfizer-BioNTech-Fosun, and the same vaccine will begin to be administered in Croatia, according to announcements issued by the Croatian Institute of Public Health. The official name of the vaccine is COVID-19 mRNA Vaccine BNT162b2 concentrate for solution for injection, but for the sake of simplicity it is called the Pfizer vaccine, although it was developed by the German company BioNTech, and the Shanghai company Fuson Pharma participated financially and to some extent in the clinical trials.

It is the first COVID-19 vaccine to be approved for use (this week in the United Kingdom and Canada, and is expected to be approved next week in the United States) and the first vaccine for a human disease ever to be developed using the new platform of messenger RNA (mRNA). Therefore, around this vaccine, as well as other COVID-19 vaccines developed using similar (mRNA) or different platforms (adenovirus vector, protein subunit, inactivated or attenuated whole virion), knowledge and ignorance have often become entwined, without a genuine desire to clearly define facts clearly and objectively consider the unknowns.  

The following is a brief presentation of the currently available facts.

  1. The question of whether the mRNA in the COVID-19 vaccine will become incorporated into the human genome. mRNA does not enter the DNA and there is no danger that injection with mRNA will modify the human genome. mRNA is translated into protein, in this instance the SARS-CoV-2 spike protein. Two amino acids in this protein are further modified (i.e., two nucleotides on the mRNA are modified), causing a conformational protein adjustment that induces high levels of neutralizing antibodies.  
  2. Long-term consequences of vaccines using mRNA platforms. It will only be possible to answer this question when a sufficient number of vaccinated persons have been observed over an extended period. However, we have long known that mRNA disintegrates very quickly; after several cycles of mRNA translation into a protein by a ribosome, the mRNA disintegrates. This takes from several hours to several days in eukaryotic cells. There is no reverse process of copying information from mRNA into DNA and then incorporating it in a genome. For now, the evident undesirable side effects of the vaccine are local (swelling and pain at the injection site) and general (fever, chills, headache and myalgia), which appear within 48 hours of vaccination, as with most other vaccines.  
  3. The issue of autoimmune diseases. This vaccine does not have an adjuvant (an agent that enhances the immune reaction), so its use cannot trigger autoimmune diseases. From the information contained in the mRNA, the coronavirus spike protein is made, which provokes a specific humoral and cellular immune response that is most accurately measured by a neutralizing antibody assay. These antibodies are crucial in the prevention of infection. To date, in Phase 3 clinical trials involving over 50,000 participants who received the Pfizer vaccine, there were no autoimmune adverse effects during the active follow-up period of at least four months. In immunology, the dynamics of an eventual autoimmune process are known: the first phase of sensitization to an antigen takes from 1 to 4 weeks, followed by inflammatory cell or tissue damage that can lead to the clinical picture of a disease. As a rule, this lasts about 4 to 6 months. So far, there have been no indications that this was experienced by any of the participants in Phase 3 of the clinical trial. 
  4. The question of the ethics of the manufacturing process. Embryonic cells from aborted children are neither used in the production nor testing of the mRNA vaccine, and they contain no components of human origin. Very simply, they contain 30 mcg of mRNA obtained in vitro by molecular engineering, water, K+ and Na+ salts and a lipid consisting of ALC-0315 and ALC-0159 plus cholesterol and another lipid molecule. These lipids encase the mRNA in lipid nanoparticles of approximately 0.1 micron (100 nanometers), which, when injected into tissue, merge with the lipophilic parts of the cell walls and thus enter the cytoplasm. The volume for the injection of this amount (1 dose of 30 mcg) of mRNA is miniscule, only 0.3 ml of mRNA and adenovirus vector vaccines are not the same. The production of the vaccine manufactured by AstraZeneca (AZD1222, colloquially known as the Oxford vaccine) involves human embryonic cells. This is ethically debatable and we are cite a statement issued by the Pontifical Academy for Life (2005), “Moral Reflections on Vaccines Prepared from Cells Derived from Aborted Human Foetuses” (available in English on various websites:, and in Croatian on our website: In 2016, the “Branimir Richter” branch of the Croatian Catholic Medical Society organized a lecture that was given by our colleague, the epidemiologist Dr. Hrvoje Vranješ,  entitled “Ethical Dilemmas in the Production and Administration of Vaccines” (available in Croatian: ). From the professional viewpoint, the Pfizer-BioNTech and AZD1222 vaccines should not be confused with each other. The former is an mRNA vaccine and the latter is an adenovirus vector vaccine, as is the Russian vaccine Sputnik V, which was already widely in use after Phase 2 of the clinical trial, and the Chinese are on track to place another adenovirus-vectored vaccine into circulation. Bypassing the Phase 3 trial is neither permitted in the European Union nor the United States before the final licensure of the product. All vaccines are required to present Phase 3 data to the regulatory agencies if they intend to be licensed in the EU, USA, Canada etc. Vaccines from Pfizer, Moderna, Astra-Zeneca and other companies based in the EU or USA that make vaccines for COVID-19 are complying with these requirements. 
  5. Adverse reactions that were not recorded during Phase 3 clinical trials. At the beginning of the vaccination campaign in England (Pfizer vaccine), several vaccinees developed allergic reactions but were found to have suffered from severe allergies previously. Therefore, it was immediately stipulated that those known to be allergic to food, vaccines or medications should not be vaccinated. For now, it is assumed that one of the lipids in the nanoparticle carrier (ALC-0315?) is responsible for such reactions.
  6. Appeals not to take vaccine but to wait. We can wait a very long time without finding out whether the doubts about the mRNA are justified if the vaccine is not administered. Calling for “caution” when all the possible precautions are already being taken is not correct. The appeal to wait and see is not correct because it entails letting others take the ”risk” while we take the position as if we were outside of the pandemic. The question is whether this is ethical.    

These lines were composed on the basis of information available to everyone. The facts have to be discerned. We shall never know everything about any drug or vaccine but this should not be a reason not to administer effective drugs or vaccines with acceptable safety profiles. So far, it does not look like the SARS-CoV-2 vaccine could have any serious medium-term consequences. However, at this point we cannot know about any eventual long-term consequences before a sufficient follow-up period has elapsed. From what we know now, we cannot tell if they should appear at all.  

In conclusion, according to the results of the clinical trials of the Pfizer and Moderna mRNA-based vaccines, persons receiving these vaccines will have 95% protection against COVID-19, with a small risk of local, and sometimes systemic, adverse reactions limited to the first 48 hours after administration. No one knows the long-term consequences. It is likely that such consequences would not last longer than the few days needed for the injected mRNA to decompose. There are certainly no ethical barriers preventing Catholics from being vaccinated with these vaccines.  

The Executive Council of the Croatian Catholic Medical Society (HKLD)