Original article (in Slovenian) was published on 10/10/2025; Author: Aljaž Primožič
Roman Jerala from the National Institute of Chemistry explained that vaccines such as those against whooping cough, influenza, and the RSV virus cannot cause complications during pregnancy.
A Facebook user who also runs a registered holistic motherhood business claimed in a 25 September post that a client of hers, a veterinarian, had given her a very particular first lesson at university.
According to her account, the lecture stated: “Pregnant animals are not vaccinated.” Based on this client’s story, the user concluded that pregnant women should not be vaccinated either. By the time of publication, 34 Facebook users had shared the post.
Tomaž Bratkovič, head of the Department of Pharmaceutical Biology at the Faculty of Pharmacy, University of Ljubljana, explained to Razkrinkavanje.si that vaccines for protecting humans and animals against infectious diseases contain one or more antigens, that is, characteristic parts of the disease-causing agent.
Vaccines fall into three main groups, he explained. Inactivated vaccines, often called killed vaccines, contain whole but destroyed pathogens or parts thereof. Such vaccines are safe for vulnerable groups, but they usually require several doses for sufficient protection.
Recombinant vaccines contain individual laboratory-created proteins of pathogens which trigger an immune response. Their advantage is a high degree of safety and precision, but the downside is that they usually trigger a somewhat weaker immune response compared to live vaccines.
The last group consists of attenuated or weakened live vaccines, which contain live but modified microorganisms. These can still reproduce but either do not cause disease or cause it in a very mild form. They effectively mimic natural infection and trigger a strong immune response. However, they are not suitable for people with weakened immune systems and pregnant women, according to Bratkovič.
Vaccination with live vaccines is generally not advisable for people living in the same household as someone with a weakened immune system: the vaccinated person sheds small amounts of the vaccine virus, which can cause infection in the immunocompromised person. “For a similar reason, we do not vaccinate pregnant women with live vaccines. There is a risk that the virus in the vaccine could be transmitted to the fetus through the placenta, leading to developmental issues during pregnancy,” he warned.
Roman Jerala, head of the Department of Synthetic Biology and Immunology at the National Institute of Chemistry, explained that the mother’s immune system changes during pregnancy such that it does not reject the fetus. The embryo, meanwhile, does not yet have a fully developed immune system and the mother’s antibodies protect it from infections. Like Bratkovič, he explained that vaccinating pregnant women with attenuated live viruses is not advisable because of the risk that the virus might reach the fetus and cause complications.
Inactivated and recombinant vaccines, however, carry no such risk, since they cannot cause infection. This is why they are recommended for pregnant women. And such vaccines have a dual effect: they protects both the mother and the fetus. Bratkovič confirmed this as well.
Jerala also pointed out that vaccines for animals are registered more quickly and tested on fewer subjects than vaccines for humans. In the approval of animal vaccines, economic interests often prevail, whereas in vaccines intended for humans, more attention is devoted to safety and monitoring of effectiveness, even after registration has been approved.
The Public Agency for Medicinal Products and Medical Devices (JAZMP) confirmed that vaccines used in human and veterinary medicine differ in physiological, ethical, and regulatory terms. Comparing different types of vaccines, or drawing conclusions based on such comparisons, is therefore not possible.
They added that medicines and vaccines must obtain authorization before being placed on the market, as required by Slovenian and European law. In the marketing authorization process, the competent authority checks their quality, safety, and effectiveness and, based on its findings, either grants or refuses approval.
The JAZMP also explained that stricter requirements apply to vaccines, since every batch must not only undergo manufacturer’s analysis but also pass independent quality control in specialized official laboratories. In Slovenia, this task is performed by the control laboratory at the National Laboratory for Health, Environment, and Food. After successfully assessing the suitability of a vaccine, the laboratory issues an official certificate that it is safe and meets quality standards.
The National Institute of Public Health (NIJZ) similarly advises that “during pregnancy, women can protect their baby against certain infectious diseases in the first months of life, when the child is most vulnerable.” Vaccination against whooping cough, RSV, and influenza is recommended, but vaccines such as those against measles and chickenpox should not be administered during pregnancy.
For these diseases, there is a theoretical risk of infecting the fetus, so vaccination should be done before pregnancy. Contracting these diseases during pregnancy can cause congenital malformations or miscarriage.
The JAZMP on the other hand, noted that there is no reliable evidence that any live vaccine administered during pregnancy causes congenital malformations in the fetus. According to their explanation, pregnant women can be vaccinated with a live vaccine in exceptional cases, but this is allowed only when the risk of infection is greater than the risk posed by vaccination.
We informed the Facebook user of our findings and will publish her response once we have received it.
The claim that pregnant women should not be vaccinated because pregnant animals are not vaccinated is manipulative. Vaccines for animals and humans are so different that they cannot be directly compared. In human medicine, it is not advisable to vaccinate pregnant women with certain types of vaccines, but vaccination itself is not discouraged.
