march. bogdan kirilov, the executive director of the bulgarian drug agency, initially stated that the country intended to continue the vaccination process as planned. however, on 12 march 2021, boyko borisov ordered that vaccinations with astrazeneca be halted until a written opinion is provided by ema, clearing all doubts regarding the vaccine's safety. as a result, kirilov mandated that all of the astrazeneca doses in the country are temporarily blocked from distribution and use. the decision had also been influenced by the reported fatality of a 57-year-old woman from plovdiv with multiple comorbidities shortly after vaccination. on 18 march 2021, ema released a statement that it still considers the benefit-risk balance of the vaccine to be positive. it was also determined that no thrombosis had been observed in the case of the 57-year-old bulgarian citizen. astrazeneca vaccinations recommenced in bulgaria on the next day. on 21 may 2021, ema introduced a contraindication for a second shot of astrazeneca in the case of blood clots with low blood platelets (thrombosis with thrombocytopenia syndrome) occurring after the first injection and on 11 june 2021 issued a warning against the administering of this vaccine to people with capillary leak syndrome. after a similar issue halted the distribution of johnson & johnson, on 20 april 2021, ema confirmed that it had discovered a link between the vaccine and instances of very rarely occurring unusual blood clots with low blood platelets, stating that the product information should contain such a warning while also assessing the benefit-risk balance to be in favour of the vaccine. from 25 may 2022, the time interval for children aged 12 to 17 in which they can receive a pfizer-biontech booster dose after completing their regular vaccination regimen with the same vaccine was shortened from six to three months. leading immunological laboratories in the country will be monitoring at least 1000 bulgarians for a period of two years in order to assess the nature and durability of their immunity to covid-19. a screenshot from the electronic registration system after vaccinations with oxford-astrazeneca were temporarily put to a halt. procedures relating to the vaccinations vaccination is voluntary and free for all citizens. the issuing of the vaccination certificate will also be at no cost for the citizen. the medical professionals administering vaccines are paid 10 leva per injection by the state. while the process had unofficially begun as early as december 2020, in late january 2021, many citizens started signing up for vaccinations with their general practitioners, with an online registration system beginning to function on 4 march 2021. more than 40,000 bulgarian citizens had registered by the late afternoon hours of 5 march. as of 10 march 2021, approximately 70,000 people had selected vaccination slots for the period between 11 and 31 march. on 28 may 2021, the caretaker health minister approved the templates for the eu digital covid certificate pertaining to a completed vaccine course, an incomplete vaccine course, and a completed mixed vaccine course. the people who have received vaccination certificates for immunizations administered up to 1 june will be able to obtain reissued versions in accordance with the eu requirements. in august 2021, bogdan kirilov, the executive director of the bulgarian drug agency, claimed that there is no guarantee that the vaccines will remain free for all citizens in the future. in late september 2021, the information portal coronavirus.bg began publishing data regarding the vaccination status of the people testing positive, in need of hospitalization and those succumbing to the disease. since 19 november 2021, the people who are to receive vaccines no longer have to fill out and sign an informed consent form. since 20 december 2021, bulgarian citizens as well as people with a permanent, long-term or of significant duration residency status in bulgaria who have been vaccinated abroad are eligible to receive boosters. on 1 july 2022, the online system allowing prior booking of slots for covid-19 vaccination temporarily ceased functioning. a clinic in poduyane, sofia where people can be tested for the virus and also receive covid-19 vaccines. legal aspects according to maria sharkova, an attorney of medical law, and todor kapitanov, an expert in the field of employment law, as well as other legal practitioners in bulgaria, employers currently cannot impose a requirement on their employees to be vaccinated against covid-19. sharkova stated in september 2021 that the administering of a booster dose could be considered to be off-label prior to any official statement made by ema and entail risks of legal repercussions for the medical practitioners who do so, especially in the case of unwelcome side effects, unless it was legally mandated by the health minister's signature. the ministry of health insisted that the green light given by the expert council on health matters in bulgaria should be sufficient. in august 2021, prior to the introduction of the vaccine passes on the national level, the medical university in plovdiv began requiring a vaccine certificate, a negative test or proof of recovery from an infection for the students who attend practical sessions. in a letter addressed to the media one of the students contested the constitutionality of this requirement. in the autumn of 2021, a number of other universities in the country that did not intend to make the full switch to remote teaching also followed suit with the same precondition for attendance. legal experts have warned that the muddled language employed in a lot of the regulations pertaining to the vaccination campaign may make it very difficult to hold the medical practitioners who provide fraudulent vaccine certificates to account. in january 2022, it was reported that bulgaria is being sued for "vaccination failure", with the bulgarian helsinki committee and the institute for european politics "open society" (bulgarian: институт по европейски политики "отворено общество") submitting complaints to the european committee of social rights and the sofia municipal court. this is due to the country failing to prioritize people with chronic conditions and the elderly between december 2020 and may 2021 in terms of providing them with access to vaccines as well as not doing enough to inform such groups of the benefits of vaccination. 36.9% of bulgarians aged over 60 were fully vaccinated at the time. effects on the pandemic estimates regarding immunity within the population the officially registered covid-19 cases in bulgaria are believed to underreport the actual number of infections by a factor of at least 2, mathematician lachezar tomov surmising that the underestimation is sixfold, with a significant amount of hidden covid-19 afflictions consistently eluding detection, in part due to people buying covid-19 test kits from pharmacies and not reporting to health authorities. in the autumn of 2020, mathematicians such as nikolay vitanov multiplied the officially registered cases by 8 while since 2021 they have been multiplying them by 4 in order to get a clearer picture regarding the real epidemiological situation. in the summer of 2022, mathematician ognyan kunchev estimated that the real number of cases may be potentially up to 10 times higher than the officially registered ones due to the low levels of testing and the transmissibility of omicron. an epidemiological study from june 2020 reached the conclusion that 2% of bulgarians had developed antibodies against covid-19. the cibalab private laboratory analyzed 2665 samples collected throughout the country between 25 may and 11 june 2021, with 1158 from them (43%) showing a positive result for the presence of antibodies. this survey has been characterized as falling far short of standards for representativeness and as methodologically suspect, with the health ministry criticized by experts for publicizing the results. alpha research sociologists speculated, based on the results of a survey for the period between 11 and 18 december 2020, that 670,000 bulgarians may have had the virus and recovered from it. in february 2021, a gallup survey conducted between the 4th and the 12th of that month drew the conclusion that 1.5 million bulgarians may have had the virus. in an april 2021 gallup survey, close to one third of adult bulgarians claimed to have either recovered from the virus or been vaccinated with two doses. мathematician nikolay vitanov estimated in late january 2021 that some 20% of bulgarians may have encountered the virus in some form. in march 2021, physician emil hristov, drawing on various sources, provided an even higher estimate of between 2 and 2.5 million while also expressing skepticism that the number of vaccinated will start to exceed those who have recovered from infection any time soon. mathematician lachezar tomov claimed in march 2021 that any numbers in the vicinity of 2 million are most likely an overestimate, though he stated that they were reached six months later - by september 2021. in late april 2021, vitanov estimated a "shield" (recovered and vaccinated) of 33.6% bulgarians against the virus. in september 2021, taking into account the likelihood of reinfections, lachezar tomov assessed the "shield" to be around 40%. according to nedyalko kalachev, the head of the cibalab laboratories, 12 024 out of 26 425 (45%) of the people who submitted samples for the month of november 2021 had igg antibody levels above 150 bau/ml (required for a green certificate unless the person has been vaccinated or has a negative test result), though he considers the truly protective threshold to be above 700 bau/ml. bulgaria will receive methodology from the eu for assessing the situation in the country in terms of natural and vaccine-generated immunity to the virus. vaccine effectiveness according to health inspector angel kunchev 93.8% of the new covid-19 infections during the third week of august 2021 were among non-vaccinated people. kunchev later announced that between 22 and 28 august 2021, 97% of covid-19 fatal outcomes involved non-vaccinated bulgarians. data provided by the ministry of health for the period between 1 march and 14 september 2021 indicated that 99.33% of the covid-19 related fatalities in the country were among people who had not been vaccinated. in the autumn of 2021, ventsislav mutafchiyski stated that over 90% of the covid-19 patients in the military medical academy were not vaccinated, with over 98% of the cases with a lethal outcome being of non-vaccinated people. herd effect projections according to experts, taking into account the situation in early 2021, 60% vaccinated people plus those who have had the disease and recovered from it may be sufficient for herd immunity to kick in. then health minister kostadin angelov stated in february 2021 that at least 70% will need to be vaccinated in order to guarantee the return of normal life. in may 2021, virologist radostina aleksandrova, taking into account the different mutations and the delay in vaccinations, tentatively calculated the herd immunity threshold to be at 80% while in march mathematician lachezar tomov put the number at 85%, assuming the alpha variant would become dominant in the country. in july 2021, molecular biologist georgi marinov stated that herd immunity could be achieved if 90% of the people are immunized, though he also characterized it as likely a temporary state of affairs due to the waning immunity from the vaccines. he thus advised the bulgarians immunized in december 2020 and january 2021 to already consider a second vaccination cycle. asen baltov, the director of pirogov hospital, cautioned in march 2021 that at least 3 million people will have to be vaccinated by august–september if the country is to avoid a fourth wave in the autumn of 2021, with caretaker health minister stoycho katsarov also acknowledging in june 2021 that bulgaria is facing a real risk of such a scenario occurring in case of insufficient vaccination rate and due to remaining issues in the health system. mathematician lachezar tomov stated in mid june 2021 that at least 30,000 new people will need to be vaccinated per day (until 15 september 2021) in order for the country to prevent this eventuality. in late april 2021, physician nikolay branzalov, taking into account the ratio of vaccinated and recovered to the newly infected, predicted that unless there are major virus mutations, escaping the vaccines, there will not be a next covid-19 wave in bulgaria. ognyan kunchev, a member of the informatics and mathematics institute of bas, estimated in march 2021 that the pandemic will peter out by the end of 2021 if 100,000-120,000 people are vaccinated per month, viewing it as likely