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Introduction
More than two years after the onset of the COVID-19 pandemic, the world continues to face challenges in overcoming the public health crisis caused by the high morbidity and mortality of SARS-CoV-2 infection1,2. Accordingly, the scientific community has unified efforts in developing and evaluating pharmacological agents to manage COVID-193. In the same sense, the development of different vaccine candidates against COVID-19 seeks to guarantee a safe immunization with good effectiveness to reduce the virus's transmission, infection, and severity4,5.
Development of COVID-19 vaccines
At the time of writing (February 6, 2021), the World Health Organization (WHO) has registered 335 candidate vaccines against COVID-19, of which 141 are in the clinical phase, 31 (21.1%) in phase 3 and 10 (7.8%) in phase 4 clinical trials6. Although decades ago, this vaccine development process required years of work and research, today, thanks to technological advances and knowledge about vaccine development7, the accelerated manufacture of several candidates has been achieved8,9, with sufficient evidence of short-term safety and efficacy to be approved early and administered to the general population10.
Developed countries have led the development of vaccine candidates against COVID-19 and have been the first to participate in clinical trials to ensure safety and efficacy11. The first vaccines were developed from different components and novel active ingredients12, such as those based on DNA and RNA (Table 1)6. The first to reach phase 3 clinical trials (in July 2020) were messenger RNA-based vaccines (Moderna and BioNTech/Pfizer)13,14, preliminarily showing superior efficacy to other vaccine types such as Sinopharm's vaccine containing inactivated virus (classical structure for vaccine manufacture)15. Despite this, regardless of the active principle of the different vaccines that reached phase 4, the efficacy shown both in the studies and in the current context has been very encouraging, since they generated a high degree of protection against infection, as well as a significant impact on the reduction of severe cases10.
Table 1 Structure of COVID-19 vaccine candidates in the clinical phase.
| Description of the structure of the COVID-19 vaccine candidates | Candidates in clinical phase n=141 (100%) | In Phase 3 n=31 (100%) | In Phase 4 n=10 (100%) |
|---|---|---|---|
| Protein subunit | 47 (33.3) | 15 (48.4) | 1 (10) |
| RNA based vaccine | 23 (16.3) | 3 (9.7) | 3 (30.0) |
| Viral Vector (non-replicating) | 19 (13.5) | 2 (6.5) | 3 (30.0) |
| Inactivated Virus | 20 (14.2) | 6 (19.4) | 3 (30.0) |
| DNA based vaccine | 16 (11.3) | 2 (6.5) | 0 (0.0) |
| Virus Like Particle | 6 (4.3) | 1 (3.2) | 0 (0.0) |
| Viral Vector (replicating) | 4 (2.8) | 1 (3.2) | 0 (0.0) |
| Live Attenuated Virus | 2 (1.4) | 1 (3.2) | 0 (0.0) |
| Viral Vector (replicating) + Antigen Presenting Cell | 2 (1.4) | 0 (0.0) | 0 (0.0) |
| Viral Vector (non-replicating) + Antigen Presenting Cell | 1 (0.7) | 0 (0.0) | 0 (0.0) |
| Bacterial antigen-spore expression vector | 1 (0.7) | 0 (0.0) | 0 (0.0) |
DNA Deoxyribonucleic Acid, RNA Ribonucleic Acid. Last updated on February 6, 2022. Source: Modified from World Health Organization (WHO). Draft landscape and tracker of COVID-19 candidate vaccines. 2020. Available at: https://www.who.int/publications/rn/item/draft-landscape-of-covid-19-candidate-vaccines
All countries faced the challenge of acquiring sufficient doses as soon as possible to vaccinate their populations against COVID-1916. However, low- and middle-income countries faced more significant difficulties in the procurement, distribution, and delivery of vaccines17. These countries, due to their scarce resources to invest in the massive and anticipated purchase of the different vaccines, could have gone through a complex process of evaluation as to which candidate vaccine would have optimal safety margins and which would also have proven efficacy since a lousy investment could put populations at risk of receiving a vaccine without the desired safety or efficacy, or that not all the population would be vaccinated due to distrust in vaccines without adequate support for their application18.
Vaccine procurement has been a cornerstone for controlling the pandemic8. Unfortunately, however, the countries of Europe, the United States, Australia and parts of Asia, in addition to having been the countries that had been developing several of the candidate vaccines, had also prepaid for the reservation of millions of doses of vaccines against COVID-19, in order to guarantee prompt vaccination. That led to an inequality in the acquisition of vaccines, since by August 24, 2020, before the reports on vaccines safety and effectiveness, these countries had already purchased 2000 million doses of vaccine16. Meanwhile, middle and low-income countries, such as those in Latin America, had to make a competitive purchase, where the first million doses were already destined for richer countries16,19.
In this context, while low-risk individuals in developed countries received some vaccine against COVID-19, health care workers in many low- and middle-income countries had no vaccine and no reasonable date for immunization12. Faced with this inequity, the Vaccine Alliance (GAVI), the Coalition for Epidemic Preparedness Innovation (CEPI5), and WHO created the Global Access to Vaccines mechanism (COVAX)12, which aimed to accelerate vaccine development and ensure fair, transparent, and equitable access, pledging 2 billion doses of vaccines to protect vulnerable and high-risk people and frontline health workers in less wealthy countries20. However, after the first year since the creation of COVAX, the goals set were not met, distributing, by June 2021, less than 4% of the vaccines administered worldwide21. As a result, low- and middle-income countries, including Latin American countries, which relied heavily on the promise of equitable and timely distribution by COVAAX, had the insufficient acquisition of doses to immunize their populations21.
A final point concerning vaccine procurement in Latin American countries is that countries such as Mexico, Cuba, and Brazil have advanced avidly developing their vaccine despite limited local vaccine production capacity19. Likewise, Argentina, Chile, Ecuador, and Peru are also facing the challenge of manufacturing their vaccine with limited progress22. However, the possibility of initiating the industrial manufacture of vaccines by their means in most Latin American countries is still a distant scenario. Consequently, the acquisition of doses will depend for the time being on the market in other countries, as well as on multinational donations23,24. On the other hand, Argentina and Mexico have joined efforts to boost local productivity by sharing the manufacturing functions of approved vaccines (vaccine production in Argentina and packaging in Mexico). However, there have been delays in obtaining the number of vaccines proposed by this initiative25.
Vaccine distribution
Another cornerstone for pandemic control with COVID-19 vaccines is their equitable and timely distribution. However, its realization represents a major logistical challenge for many developing countries24. Therefore, pre-arrangements are required for vaccine distribution26. In addition, it is necessary to generate an environment conducive to safety and trust in vaccines since the population may have doubts and false perceptions that could lead to the misuse of resources to conserve and distribute vaccines27. Therefore, it is necessary to combat misinformation and propose active surveillance during vaccination to provide more excellent safety to the population28.
In each country, health authorities have prepared strategic prioritization plans29, offering the vaccines first to healthcare workers and persons at high risk of severe COVID-19, and finally concluding with immunization of the rest of the population30. However, in the middle- and low-income countries, for vaccines distribution, the logistics needed to stockpile different types of COVID-19 vaccines had to be anticipated6, since, as with other vaccines, adequate local transport capacity will be required for different communities, ensuring adequate storage until the dose is administered, the availability of associated and necessary materials for vaccination (needles, syringes and diluents), equipment in local facilities to preserve the efficacy and safety of the biological product, as well as comprehensive training for immunizers and workers in the vaccination centers31. In addition, due to the different specifications for the conservation of each type of vaccine against COVID-19, countries with limited equipment or resources to meet these requirements in all their cities had to evaluate the type of vaccine they wanted to acquire since some of them require temperatures as low as -80°C32. Unfortunately, these conditions are not all cities, limiting equitable distribution.
Number of doses of the SARS-CoV-2 vaccines
Another point that low-income countries had to consider before acquiring any of the vaccines against COVID-19 was the number of doses that would need to be administered to their population to achieve immunity since the number of doses required varied among the different vaccines6. Therefore, when a vaccine entered the vaccination schedule of the countries, a continuous effort was needed to comply with the scheduled dates for administering the biologic and to guarantee adequate immunogenicity in the population (24). In low- and middle-income countries, where the acquisition of doses is limited, scientific evidence indicated that it was more advisable to immunize the most significant number of people with one or two doses than to offer many more administrations to only some groups; this is because some of the vaccines (Oxford/AstraZeneca and BioNTech/Pfizer), demonstrated that a single dose developed sufficient levels of antibodies to neutralize SARS-CoV-2, and thus reduce the risk of developing severe COVID-1933. Nevertheless, applying the complete scheme (usually two doses) was of utmost importance to significantly reduce the incidence and mortality from COVID-19.
On the other hand, applying a booster (third or fourth dose) seems to increase and enhance immunogenicity against COVID-19 infection34. Therefore, an attempt should be made to reach a higher proportion of vaccinated persons with the two or three doses and subsequently guarantee the administration of a booster, prioritizing those groups with a higher risk of severe disease. Furthermore, although, for the time being, more solid studies are needed on the average time in which the doses administered should be spaced apart from the booster, countries should guarantee a more significant number of doses in case it is necessary to include the COVID-19 vaccine in the annual immunization schedule. Unfortunately, although this strategy could be a viable option for those countries that have sufficient vaccines, for many Latin American countries, this scenario does not seem possible in the short term, since many of these countries have barely managed to acquire enough doses to vaccinate one or two rounds of their total population.
SARS-CoV-2 strains and vaccine efficacy
Multiple variants of SARS-CoV-2 have circulated worldwide (Table 2)35. Each variant was characterized by mutations in its structure, mainly in the Spike protein, which favours virus binding to human cells36. In addition, the epidemiological consequences of the appearance of new SARS-CoV-2 variants were recorded based on the transmission potential, infectivity, pathogenicity and lethality of the virus37.
Table 2 SARS-CoV-2 Variants and attributes towards vaccination.
| WHO label | PANGO lineage | Earliest documented samples | Variant classifications | Attributes of transmissibility | Attributes towards vaccination |
|---|---|---|---|---|---|
| Latin American variants | |||||
| Gamma | P.1 | Brazil, November-2020 | Variant of Concern | Not assessed | Reduced neutralization by convalescent and postvaccination sera. |
| Zeta | P.2 | Brazil, April-2020 | Formerly monitored variants | Not assessed | Reduced neutralization by postvaccination sera. |
| Lambda | C.37 | Peru, December-2020 | Variant of Interest | Not assessed | Not assessed |
| Mu | B.1.621 | Colombia, January-2021 | Variant of Interest | Not assessed | Not assessed |
| Other variants | |||||
| Alpha | B.1.1.7 | United Kingdom, September-2020 | Variant of Concern | Around 50% increased transmission. Potential increased severity based on hospitalizations and case-fatality rates. | Minimal impact on neutralization by convalescent and postvaccination sera. |
| Beta | B.1.351 | South Africa, May-2020 | Variant of Concern | Around 50% increased transmission. Potential increased severity based on hospitalizations and case-fatality rates. | Reduced neutralization by convalescent and postvaccination sera. |
| Delta | B.1.617.2 | India, October-2020 | Variant of Concern | Increased transmissibility. | Reduced neutralization by convalescent and postvaccination sera. |
| Epsilon | B.1.427 / B.1.429 | United States-(California), March-2020 | Formerly monitored variants | Around 20% increased transmission. | Reduced neutralization by convalescent and postvaccination sera. |
| Eta | B.1.525 | United Kingdom/ Nigeria, December-2020 | Formerly monitored variants | Not assessed | Potential reduction in neutralization by convalescent and post-vaccination sera. |
| Iota | B.1.526 | United States (New York), November-2020 | Formerly monitored variants | Not assessed | Reduced neutralization by convalescent and postvaccination sera. |
| Kappa | B.1.617.1 | India, October-2020 | Formerly monitored variants | Not assessed | Reduced neutralization by convalescent and postvaccination sera. |
| Omicron | B.1.1.529 | Multiple countries, November-2021 | Variants of concern | Increase in transmissibility. | Reduced neutralization post-vaccination sera. |
| Theta | P.3 | Philippines, January-2021 | Formerly monitored variants | Not assessed | Not assessed |
Last updated on February 6, 2022. PANGO: Phylogenetic Assignment of Named Global Outbreak. Variant of Interest: A variant with specific genetic markers that have been associated with changes to receptor binding, reduced neutralization by antibodies generated against previous infection or vaccination, reduced efficacy of treatments, potential diagnostic impact, or predicted increase in transmissibility or disease severity. Variant of Concern: A variant with evidence of an increase in transmissibility, more severe disease (e.g., increased hospitalizations or deaths), a significant reduction in neutralization by antibodies generated during previous infection or vaccination, reduced effectiveness of treatments or vaccines, or diagnostic detection failures. Formerly monitored variants: The variant is no longer circulating at global public health significance levels. However, the variant has been circulating for a long time without impacting the overall epidemiological situation, or scientific evidence demonstrates that the variant is not associated with any concerning properties. Source: Health Organization (WHO) extracted and modified data regarding SARS-CoV-2 variants, and their attributes were extracted and modified from Health Organization (WHO). Tracking SARS-CoV-2 variants [Internet]. 2021 [cited 2022 February 6]. Available from: https://www.who.int/en/activities/tracking-SARS-CoV-2-variants/ and Centers for Disease Control and Prevention. SARS-CoV-2 Variant Classifications and Definitions [Internet]. 2021 [cited 2022 February 6]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-info.html.
In Latin America, several strains (Gamma, Zeta, Lambda, Mu) were identified as variants of interest or concern due to their potential risk of transmissibility and evasion of vaccine neutralization35. However, Latin countries were mainly confronted with variants initially identified in other regions. For example, the Delta variant (B.1.617.2) identified in India35 was possibly responsible for a wave during the end of 2021, in which there was an accelerated increase in COVID-19 cases38 because the variant had higher transmissibility (about 60% higher than the alpha variant) and was moderately resistant to vaccines, especially in people who had received a single dose39. Similarly, the Omicron variant, identified in multiple countries, has been responsible for a new wave in Latin American countries, significantly impacting public health due to its high transmissibility and ability to infect immunized persons40. In addition, this variant presented little or no neutralization capacity after a series of 2 doses of messenger RNA vaccines41. However, with the application of a booster dose, increased protection against symptomatic or asymptomatic infections, transmission and severe forms were observed42.
Antigen mutation in SARS-CoV-2 variants was a due process since it was previously observed in other coronaviruses43. This ability to mutate puts the efficacy of vaccines at risk because they may become less neutralizable in the future37. Despite this, to date, messenger RNA-based vaccines appear to maintain efficacy against coronavirus variants44. However, it will still be necessary to re-evaluate the efficacy of each vaccine against the emergence of new variants45, taking into consideration that populations where vaccination is not massive, could generate an environment with conditions to favour new variants. Therefore, strains should be monitored in all regions, and vaccine efficacy should be evaluated periodically43.
Impact of vaccines on the pandemic
By November 2, 2021, 49.6% of the world's population had received at least one dose of a COVID-19 vaccine, with most doses administered in China, the United States, and India46. February 6, 2022, this figure has increased to 61.5%, with 10.240 billion doses administered globally46. Following the application of COVID-19 vaccines, it has been shown to reduce mortality in all age groups that received it47. However, even the efforts to achieve herd immunity are insufficient since massive and simultaneous vaccination of all world regions is required16.
Vaccination against COVID-19 has shown that socioeconomic inequalities play an essential role in guaranteeing the population's health, being responsible for the fact that many regions have not been adequately covered in the present pandemic. That could delay the goal of achieving herd immunity at the global level, according to estimates for 2023 or 20249,48. Despite this bleak scenario, it must be recognized that the natural immunity achieved after COVID-19 infection and the advancement of vaccination has contributed to the acquisition of herd immunity in populations. Therefore, protecting vulnerable individuals from severe outcomes remains crucial as the virus mutates and becomes endemic49.
Current status of acquisition and coverage in Latin America
Currently, Latin American countries are still in negotiations to acquire a vaccine against COVID-19 since, due to their limited capacity to produce their vaccine in a short time, the acquisition of doses depends on the market19,24, a situation that leads to inequity in achieving vaccine coverage in comparison with richer countries11.
By February 6, 2022, the total number of doses acquired in Latin America was sufficient to immunize at least twice the entire Latin American population (1,483,932,255 vaccines acquired for a population of approximately 611,289,000); however, inequities within this region give rise to a scenario in which not all countries have sufficient doses. Two-thirds of the doses were distributed in Brazil (40.8%), Mexico (14.7%), Argentina (13.2%) and Peru (11.7%)16. Three of the 21 countries (Honduras, Nicaragua and Venezuela) did not have enough doses to cover 100% of their population with the first dose of vaccine (Table 3).
Table 3 Acquisition and Vaccine Coverage against COVID-19 in Latin America
| Number of confirmed purchased | Total confirmed purchased doses | Number of doses of vaccine from donations | Total doses acquired | Total population | Potential coverage of a first dose (%) | COVID-19 vaccine doses administered | Potential people who could have received a first dose (%) | |
|---|---|---|---|---|---|---|---|---|
| North America | ||||||||
| Mexico | Pfizer BNT162 (34 400 000), Oxford University AZD1222 (79 430 000), Gamaleya Research Institute Sputnik V (24 000 000), Sinovac Coronavac (20 000 000), Sinopharm (12 000 000), CanSino Biologics Ad5-nCoV (35 000 000). | 204 830 000 | 13 110 900 | 217 940 900 | 127 950 000 | 170.3 | 169 630 000 | 132.6 |
| Central America | ||||||||
| El Salvador | Pfizer BNT162 (4 400 000), Oxford University AZD1222 (2 000 000), Sinovac Coronavac (2 000 000). | 8 400 000 | 6439 370 | 14 839 370 | 6 826 000 | 217.4 | 9 980 000 | 146.2 |
| Panama* | Pfizer BNT162 (5 000 000). | 5 000 000 | 1 503 450 | 6503450 | 4 339 000 | 149.9 | 6 660 000 | 153.5 |
| Costa Rica* | Pfizer BNT162 (4 000 075), Oxford University AZD1222 (1 000 000). | 5 000 075 | 2 029 820 | 7 029 895 | 5 163 000 | 136.2 | 8 320 000 | 161.1 |
| Guatemala | Gamaleya Research Institute Sputnik V (8 000 000). | 8 000 000 | 13 835 780 | 21 835 780 | 17 110 000 | 127.6 | 13 720 000 | 80.2 |
| Honduras* | Pfizer BNT162 (2 700 000), Oxford University AZD1222 (1 400 000), Gamaleya Research Institute Sputnik V (70 000). | 4 170 000 | 4 695 780 | 8 865 780 | 9451 000 | 93.8 | 10 570 000 | 111.8 |
| Belize | Unknown | Unknown | 723 150 | 723 150 | 430 000 | 168.2 | 445 691 | 103.6 |
| Nicaragua* | Unknown | Unknown | 5 641 130 | 5 641 130 | 6 665 000 | 84.6 | 8 880 000 | 133.2 |
| South America | ||||||||
| Chile | Pfizer BNT162 (10 000 000), Oxford University AZD1222 (14 400 000), Janssen (J&J) Ad26,CoV2,S (4 000 000), Sinovac Coronavac (60 000 000), CanSino Biologics Ad5-nCoV (1 800 000). | 90 200 000 | Unknown | 90 200 000 | 19 679 000 | 458.4 | 46 890 000 | 238.3 |
| Argentina | Pfizer BNT162 (20 000 000), Oxford University AZD1222 (23 600 000), Gamaleya Research Institute Sputnik V (30 000 000), Moderna mRNA-1273 (20 000 000), Sinopharm (34 000 000), CanSino Biologics ad5-nCoV (5 400 000). | 133 000 000 | 6 2530 920 | 195 530 920 | 45 809 000 | 426.8 | 89 020 000 | 194.3 |
| Brazil | Pfizer BNT162 (300 000 000), Oxford University AZD1222 (102 000 000), Janssen (J&J) Ad26. COV2.S (38 000 000), Sinovac Coronavac (100 000 000), CanSino Biologics ad5-nCoV (60 000 000). | 600 000 000 | 5 216 600 | 605 216 600 | 212 897 000 | 284.3 | 369 520 000 | 173.6 |
| Peru | Pfizer BNT162 (67 000 000), Oxford University AZD1222 (14 000 000), Gamaleya Research Institute Sputnik V (20 000 000), Moderna (20 000 000), Sinopharm (49 000 000). | 170 000 000 | 3416 130 | 173 416 130 | 33 035 000 | 524.9 | 56 250 000 | 170.3 |
| Bolivia | Oxford University AZD1222 (5 000 000), Gamaleya Research Institute sputnik V (2 600 000), Sinopharm (1 400 000). | 9 000 000 | 12 188 190 | 21 188 190 | 11 797 000 | 179.6 | 12 020 000 | 101.9 |
| Ecuador* | Pfizer BNT162 (6 000 000), Oxford University AZD1222 (5 000 000), COVAXX -United Biomedical-UB-162 (2 000 000), Sinovac Coronavac (2 000 000), CanSino Biologics ad5-nCoV (6 000 000). | 21 000 000 | 4 248 350 | 25 248 350 | 17 794 000 | 141.9 | 30 310 000 | 170.3 |
| Guyana | Gamaleya Research Institute Sputnik V (800 000), Sinopharm (100 000). | 900 000 | 572 790 | 1 472 790 | 790 000 | 186.4 | 779 882 | 98.7 |
| Uruguay* | Pfizer BNT162 (2 000 000), Sinovac Coronavac (1 700 000) | 3 700 000 | 500 000 | 4 200 000 | 3 555 000 | 118.1 | 7 490 000 | 210.7 |
| Colombia* | Pfizer BNT162 (10 000 000), Oxford University AZD1222 (10 000 000), Moderna mRNA-1273 (10 000 000), Janssen (J&J) Ad26,CoV2,S (9 000 000), Sinovac Coronavac (2 000 000) | 49 000 000 | 15 561 270 | 64 561 270 | 51 049 000 | 126.5 | 72 930 000 | 142.9 |
| Venezuela* | Gamaleya Research Institute Sputnik V (10 000 000) | 10 000 000 | 500 000 | 10 500 000 | 28 705 000 | 36.6 | 35 350 000 | 123.1 |
| Paraguay | Pfizer BNT162 (1 000 000), Moderna mRNA-1273 (2 000 000), Bharat Biotech - COVAXIN (600 000), Sinopharm (250 000). | 3 850 000 | 4 212 350 | 8 062 350 | 7 353 000 | 109.6 | 7 670 000 | 104.3 |
| Suriname | Unknown | Unknown | 956 200 | 956 200 | 592 000 | 161.5 | 498 165 | 84.1 |
| French Guiana | Unknown | Unknown | Unknown | Unknown | 300 000 | Unknown | Unknown | Unknown |
| Total (Latin America) | 1 326 050 075 | 157 882 180 | 1 483 932 255 | 611 289 000 | 242.8 | 956 933 738 | 156.5 | |
Last updated on February 6, 2022.
* In Countries with underreporting in the number of doses acquired, the proportion of vaccination is higher than the number of doses.
The estimated potential vaccination coverage for the first dose was determined by the percentage of a country's total population vaccinated with the total doses acquired to date. Therefore, the doses administered represent the number of doses administered to the population without considering that a person could have received more than one dose. Source: Data underlying total country doses purchased of COVID-19 vaccines were extracted and modified from Duke Global Health Innovation Center. Tracking COVID-19 Vaccine Purchases Across the Globe | Launch and Scale Speedometer [Internet]. 2021 [cited 2022 February 6]. Available from: https://launchandscalefaster.org/covid-19/vaccinepurchases; data on the number of COVID-19 vaccine doses administered were extracted and modified from Our World in Data. COVID-19 vaccine doses administered [Internet]. 2021 [cited 2022 February 6]. Available from: https://ourworldindata.org/grapher/cumulative-covid-vaccinations
Regarding the application of COVID-19 vaccines in Latin American countries, it is observed that by the date (February 6, 2022), Uruguay (210.7%) and Chile (238.3%) have administered double the number of doses concerning their population. Meanwhile, Argentina (194.3%), Brazil (173.6%), Ecuador (170.3%), Peru (170.3%), Costa Rica (161.1%), and Panama (153.5%) are on track to achieve this goal. However, this leaves the rest of the countries (13 countries) with the need to intensify their vaccination strategies (Table 3).
Another factor that limited immediate distribution to all cities in Latin American countries was the limited logistics for the proper conservation of vaccines. Some vaccines required special storage conditions to break the cold chain49. For example, the ChAdOx1 nCoV-19 vaccine from Oxford University/AstraZeneca (United Kingdom) was the most widely distributed in Latin America16. That could have been because this vaccine did not require very rigorous logistics, remaining viable at a temperature of 2 and 8 °C. On the other hand, the vaccine from Moderna (United States) and Pfizer in collaboration with BioNTech (United States), requiring temperatures below freezing (-80°C) for long-term storage, was purchased in smaller quantities, since it would limit equitable distribution to all cities23,50.
Despite all these barriers and limitations to achieving immunization of the Latino population, vaccination is progressing. In the first groups that received some new vaccines, a decrease in the mortality rate is recorded51. In addition, when waiting for a more significant number of doses to complete the vaccination schedule (two doses plus booster), the application of a heterologous dose (dose of a vaccine different from the one applied in the first dose) has been integrated as a mitigation measure against the delay in the acquisition of more vaccines52. Although there is evidence (CombiVacs study) on the efficacy of applying the Oxford-AstraZeneca vaccine with the Pfizer-BioNTech vaccine53, there is no certainty as to whether this regimen is superior to vaccination with the same type of biologic in the long term or future safety (54), so their combination should be carried out with caution and long-term surveillance55.
COVID-19 vaccine acceptance and hesitancy
Acceptance of vaccination against COVID-19 in Latin American countries has been variable (80% among Latin American citizens)56, ranging from 72 to 85.4% in Brazil (57,58), 74.9% in Peru59, 76.3% in Mexico58, and 97% in Ecuador60. However, the persistence of indecision on the part of a percentage of the population could generate gaps that would limit the achievement of collective immunity. Therefore, the dissemination of information about vaccines should be considered an additional point within national vaccination strategies to favour greater acceptance of the vaccine among those who have not yet decided to be vaccinated61. Therefore, we recommend that vaccination campaigns be accompanied by adequate information on the need for immunization against COVID-19.
Conclusions
The speed at which different SARS-CoV-2 vaccines were developed was impressive. In less than a year, a new disease has been characterized, a new viral genome has been sequenced, and the efficacy of different vaccines has been established in clinical trials. In addition, vaccine procurement, distribution, and delivery have accelerated in developed countries. Meanwhile, in low- and middle-income countries such as those in Latin America, insufficient acquisition of doses, unequal distribution of vaccines due to lack of logistic resources, and the presence of reluctance to vaccines in the population have generated slow progress in immunization in this region. Given this, strategies should be promoted to intensify local production and guarantee the acquisition of more doses, providing vaccination centres with the necessary resources to conserve the biological product and thus favour equitable distribution to the rest of the region. In addition, it is necessary to formulate measures to reduce the population's doubts and maximize the acceptance of vaccine doses and their timely boosters.
