that approximately 1 million bulgarians have already had the virus. allergist bogdan petrunov stated in march 2021 that vaccinating 2 million bulgarians may be sufficient for attaining herd immunity while mathematician lachezar tomov has insisted that approximately 5 million vaccinated will be needed for collective immunity of the population. in mid april 2021, kostadin angelov stated that in an optimistic scenario herd immunity could be attained by the end of june, with the more realistic estimate being the end of july and the most pessimistic one considered to be the end of august. dominant variants the different covid-19 variants and their interactions with natural and vaccine-induced immunity are likely to change initial calculations when it comes to the herd immunity thresholds. in june 2021, the presence of the beta and delta variants was detected by the national center for infectious and parasitic diseases in the country. the latter established itself as the dominant variant (being found in over 50% of genomic sequencing samples) as early as mid july 2021, beginning to rapidly displace alpha. delta's contagiousness and the still low vaccination coverage caused some experts to speculate that another wave may occur in bulgaria by late august, which eventually materialized as a scenario at that time. in september 2021, mathematician lachezar tomov stated that the wave driven by the alpha variant that had affected bulgaria during the spring could in part compensate for the very low level of vaccination coverage of the population, though he still expected a major rise in cases due to the beginning of the school year and the aggressiveness of the delta variant, also expressing serious concern due to the lax pandemic control measures. in late november 2021, mathematician antony rangachev stated that the introduction of the green certificates and the switch to online education for some of the students had been the major factors contributing to the turning of the tide during the fourth wave of the pandemic, though he expressed the concern that covid-19 cases in the country could remain at a high plateau. on 22 december 2021, nedyalko kalachev, the owner of the cibalab laboratories, revealed that omicron variant had been found in six virus samples. iva hristova, head of the center for infectious and parasitic diseases, stated that the presence of omicron will need to be confirmed through genomic sequencing, for which the private laboratories lack the technology, with precise findings expected to be available after the new year. in an interview on 27 december 2021, mathematician nikolay vitanov estimated that omicron is already in bulgaria, believing that cases with this variant will start to rise by mid january 2022, with a fifth virus wave occurring in february 2022 as a result of omicron's very high transmissibility and the still low vaccination coverage of the population. on 2 january 2022, the health authorities confirmed the first 12 cases of the omicron variant. 7 of the infected had not been vaccinated, 5 had been fully vaccinated and none had received a booster dose. by the second half of january 2022, omicron had displaced delta in terms of dominance in bulgaria. lachezar tomov cautioned in february 2022 that bulgaria had so far failed to reverse any negative trends with regard to the major surges of the virus by relying mainly on herd immunity and without the imposition of new measures. r0 number the r0 number for the virus is believed to be able to reach values as high as 5 or 6 if the spread is uncontrolled, rising to 7 and 9 in the case of the delta variant, with a potential reproduction factor of 15 with regard to omicron. a further caveat is that in the case of covid-19, it may not be the best measure for the state of the epidemiological situation due to the importance of superspreaders. in bulgaria the ro was estimated to be at 1.9 (with 10 people infecting 19 others) towards the end of october 2020, between 1.2 and 1.3 in early march 2021 and around 1.1 on 22 march 2021 (the first day of the third lockdown). on 13 may 2021, mathematician nikolay vitanov declared that the third wave in bulgaria is ending, emphasizing the importance of the vaccinations and appraising the basic reproduction number to be at the record low 0.595. approximately one month later, on 11 june, it stood at 0.716. the effective reproduction number reached a value of 1.7 during the first weeks of the pandemic and hovered at slightly above 1 in may 2020. it is believed to have been constantly on the rise since 14 january 2021, exceeding 1 by early february 2021. it was below 1 for the month of may 2021, being calculated to be 0.81 around 6 may 2021 and 0.80 around 6 june 2021. in mid july 2021, experts warned that the number is once again around 1, which was also ultimately assessed to be the beginning of a fourth wave, and it reached a value of 1.5 in early august. it continued to consistently exceed 1 during the next two months, being assessed to be at 1.3 around 22 october, though it dropped to below 1 during november before starting to once again hover around that number towards the end of that month. the r0 number remained at around or just below 1 during december before rising sharply and reaching 1.7 in january 2022. it subsequently fell to slightly above 1 and was assessed as stable at 0.99 towards the end of the month. it was estimated to be below to just below 1 during the whole of february 2022 and march 2022. other medical interventions the covid-19 vaccines remain the most reliable means of protection against infection, severe disease, lethal outcomes, and long-term effects of the disease. the presence of immunocompromised people within the population that may not derive the full benefits from vaccination and especially bulgaria's rather low vaccination coverage has nonetheless resulted in a continued strong focus on the potential cures for covid-19 long after the beginning of the immunization campaign, with the discovery of a highly effective and accessible medication gauged to be one of the ways to truly end the pandemic. in terms of the treatment and management of the disease, bulgaria has used, amongst other medications and depending on the severity of the symptoms, remdesivir, which first became available in june 2020, with the country having bought over 180,000 flasks from it by november 2021, and dexamethasone. ivermectin, which the bulgarian executive agency for medicines approved on 15 january 2021 for the treatment of gastrointestinal strongyloidiasis, has been prescribed off-label by some physicians, even though its use, unless it is within the scope of well-designed clinical trials, runs contrary to the advice of ema and the who, in part due to the potential for serious side effects. colchicine has also been given to patients since april 2020 and shown promise, though its effectiveness and the exact dosage to be administered is still under evaluation. in october 2021, the country ordered 3000 doses of monoclonal antibodies, which arrived in november 2021 and can be administered in hospital settings. bulgaria also signed a contract for the delivery of 20,000 doses of molnupiravir as well as unspecified quantities of paxlovid, successful clinical trials for which were held at the st. pantaleymon hospital in plovdiv in december 2021. the ministry of health has signed a contract for the procurement of kineret as well. the quality of treatment and the protocols followed have been criticized by a number of physicians and scientists on a multitude of grounds, especially due to the lack of a unified approach among physicians in managing covid-19 symptoms and practices such as the overreliance on antibiotics in the early stages of the disease, with the latter also noted as a concern by the who. the staffing-to-patient ratio for serious covid-19 cases has also been regarded as falling far short of the international standards. controversies and criticism domestic critiques of the authorities' policies the government has come under scrutiny for the delay in ordering a number of vaccines, the lack of sufficient transparency regarding the quantities, the call to give up on doses of the two rna vaccines, for which bulgaria had been eligible, during the initial pre-orders as well as the reasons behind its decision to center its vaccination campaign on oxford-astrazeneca in particular. in the case of the oxford vaccine, due to the cheaper cost of astrazeneca relative to the rna vaccines and the former's easier storage requirements, the authorities have mainly been criticized for allowing financial considerations to take precedence over health-related ones. concerns have also been raised due to the government not outlining a plan for creating special vaccination centers and training additional personnel capable of administering vaccine shots, which has contributed to a slow pace of the vaccinations. the lack of expediency with the vaccinations has been assessed as entailing significant risks for the country being able to meet its economic growth targets for 2021 and has been deemed likely (in combination with the relaxing of many pandemic control measures during the month of april 2021) to considerably lessen the country's appeal as a summer destination for tourists due to a continuing high rate of infections. as of late may 2021, only 45% of the vaccine doses delivered to bulgaria had been administered, compared to an average for the eu of 85%. in september 2021, chief health inspector angel kunchev reiterated that if the vaccination coverage remains at a low level, bulgaria will hardly see tourists visiting during 2022. pulmonologist kosta kostov has been very critical of the way in which the vaccination campaign has been organized and implemented, regarding it as not premised on empathy, being incredulous that people over 65 and clinically vulnerable individuals are in phase 4 rather than being the top priority targets and maintaining that people who have recovered from the disease should not be vaccinated as long as there is an insufficient quantity of vaccines. similarly, dimitrina petkova, an expert on equality in health care, has characterized the vaccination plan as unfair to the elderly due to attaching a greater value on reducing the spread by vaccinating economically active young people rather than by focusing on especially vulnerable groups in order to save the most lives, a sentiment echoed by caretaker health minister stoycho katsarov who considered its faulty approach in terms of prioritization and the issues with vaccine procurement to have resulted in the loss of 10 000 lives. the decision taken by the government on 19 february 2021 to set in motion the mass vaccination process has been criticized for inviting chaos, with general practitioners complaining of a lack of vaccines provided to them for their registered patients who belong mainly to high-risk groups, with most of the doses allocated to the hospitals for random vaccinations, not allowing for thorough medical check-ups of the people prior to the administering of vaccines. concerns have also been raised that the citizens who had already received one shot may not be able to get their second injection as scheduled. the shift in the modus operandi has been contested for its dubious legal basis due to being in apparent contradiction with the authorities' own phased approach to the vaccinations, entailing the prioritization of vulnerable groups such as the elderly, and has been characterized as a desperate attempt to boost the vaccination statistics. as of 1 april 2021, only 9% of the elderly and chronically ill had been vaccinated. as of 18 may 2021, less than 20% of the people considered especially vulnerable had received at least one dose of vaccine. by 24 june 2021, one third of care home residents had been administered at least one shot of vaccine. the issue has resulted in media crossfires between kostadin angelov and his caretaker government successor, stoycho katsarov. the need for general practitioners to queue for vaccines for their patients in front of the regional health inspectorates has invited tension and been described as humiliating as well as an example of an inefficient time management. in response to this issue, the caretaker government decided in may 2021 to set up logistical points for each municipality where regional health inspectorates are to deliver flasks with vaccines, taking into account the quantities requested by the general