COVID-19 Vaccination: Time for Boosting

  • Malak Kaddoura and Lubna Jaber
  • November 17, 2021

The spread of viral diseases causing respiratory illnesses continues to undermine the welfare of populations and poses a heavy threat to public health. A pneumonia outbreak, triggered by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was reported in China in December 2019 causing the coronavirus disease (COVID-19) [1,2]. This highly contagious virus initially spread across 52 countries leading to 83,652 confirmed cases in the first two months [3]. The spread of the virus continued to rise reaching 114 countries with over 118,000 cases and 4,291 deaths by March 2020, thus has been declared a pandemic [4]. By the end of 2020, approximately 4.5 million confirmed COVID-19 cases were reported [5].


COVID-19 Vaccine

The worldwide fast spread of COVID-19, along with its global impact, necessitated an urgent response from public health authorities and the development of a vaccine to constrain the virus. Consequently, 18 billion USD were provided by the US program (Operation Warp Speed) to develop the COVID-19 vaccines [6]. By December 2020, the first COVID-19 vaccine (Pfizer-BioNTech) has been approved by the Food and Drug Administration (FDA) to individuals ≥16 years old [7], and is now approved for use among children and adolescent (5-15 years) [8,9] (See image below).

Two additional vaccines were authorized for individuals above 18 years, namely Moderna and Johnson & Johnson [10]. To date, roughly 7.31 billion vaccine doses were administered worldwide with approximately 51.1% of the global population having received at least one dose [11]. However, the global vaccination coverage is quite disparate, especially between high- and low-income countries. This is also evident on a regional level; for example, in the Arab world, high-income countries like the United Arab Emirates (UAE) and Bahrain exhibited higher vaccination coverage compared to low income countries (e.g. Yemen, Syria, Sudan, etc.) (See image below).

Emerging evidence has shown that fully vaccinated individuals have better protection against SARS-CoV-2 compared with having had COVID-19. A case-control study investigating the relation between COVID-19 reinfection and COVID-19 vaccination revealed that full vaccination was associated with a lower risk of reinfection compared to non-vaccination [12]. Thus, the Central for Disease Control (CDC) recommends that all individuals must get vaccinated, despite already having been infected with SARS-CoV-2 [13].


The need for a third/booster dose

Despite the high rate of vaccination, the number of COVID-19 cases continued to increase, reaching 248,467,363 confirmed cases and 5,027,183 deaths worldwide to date [5]. The increase in cases may be associated with decreased COVID-19 restrictions, viral mutations that reduce the virus susceptibility to the newly developed vaccines, and the hints of waning immunity. In fact, a cross-sectional study of fully vaccinated adults (≥18 years) showed a gradual decline in antibody levels against SARS-CoV-2, 21 to 70 days, or more, after taking the second dose of Oxford–AstraZeneca or Pfizer–BioNTech vaccines [15]. Hence, a booster/third dose for vaccinated individuals is needed to help people maintain their immunity for a longer period of time [14]. The two terms ‘third dose’ and ‘booster shot’ correspond to the same vaccine, but the choice of terminology depends on the characteristics of the target population. The term ‘third dose’ is used when dealing with individuals with underlying medical conditions that result in sub-optimal response to primary vaccination such as the elderly or immunocompromised individuals [16,17]. The ‘booster shot’ term is used when an additional dose of vaccine is given after the protection provided by the vaccines wanes over time [19].

The use of a third dose of Pfizer-BioNTech or Moderna vaccine was first authorized by the FDA on August 12, 2021, for certain immunocompromised individuals, particularly those who have undergone solid organ transplantation or those having a comparable degree of immunocompromisation. This additional shot must be given at least 28 days after the completion of the primary vaccination series (2-dose regimen) [20]. In September and October 2021, a third dose of Pfizer-BioNTech vaccine (full dose) or Moderna vaccine (half dose), respectively, was approved to be administered at least 6 months after completing the 2-dose regimen to elderly (≥65 years). A booster shot of Pfizer-BioNTech vaccine (full dose) or Moderna vaccine (half dose) was approved to be administered at least 6 months after completing the 2-dose regimen to adults (18-64 years) at high risk of exposure to the virus and individuals who are prone to severe COVID-19 complications [21,22]. Many countries have followed this guideline. For example, in Lebanon, the booster shot vaccination campaigns started targeting elderly aged ≥75 years and healthcare workers who completed their primary vaccination series at least 6 months ago (See image below).

Moreover, on October 20, 2021, FDA authorized the emergency use of a second booster shot of the one-dose vaccine (Johnson & Johnson vaccine) for adults (≥18 years of age) to be administered at least 2 months after taking the first dose. The FDA has also authorized the ‘mix and match booster dose strategy’, whereby the booster dose can be different from the first two administered doses. For instance, an individual takes a Pfizer booster shot after taking 2 doses of Moderna Vaccine [22].

The majority of the adverse reactions to the booster were mild to moderate similar to those reported in the primary vaccination series (e.g., injection site pain, tiredness, headache and fever) [23]. However, booster doses should be administered in a timely manner to avoid the risk of harm. For instance, too early or frequent administration of COVID-19 vaccines might increase the risk of Guillain-Barre syndrome that is associated with adenoviral vector COVID19 vaccines (Astrazeneca) [24] or the immune-mediated adverse events especially with some mRNA vaccines (e.g., Pfizer-BioNTech and Moderna) [25].



The fast spread of the SARS-CoV-2 virus, along with its distinct levels of severity, urged the rapid development of vaccines to help fight the pandemic. Currently, a third COVID-19 vaccine dose is needed to ‘boost’ the immune response of the vaccinated population and extend the main vaccination course in in elderly and immunocompromised [11].


Filters: COVID-19, Vaccine





Malak Kaddoura and Lubna Jaber



[1] WHO. Pneumonia of unknown cause – China [Internet]. 2020 [cited 2021 Nov 9]. Available from:


[2] Zhu N, Zhang D, Wang W, Li X, Yang B, Song J, et al. A Novel Coronavirus from Patients with Pneumonia in China, 2019. N Engl J Med. 2020 Feb 20;382(8):727–33.


[3] WHO. Coronavirus disease 2019 (COVID-19) Situation Report – 39 [Internet]. 2020 [cited 2021 Nov 9]. Available from:


[4] WHO. WHO Director-General’s opening remarks at the media briefing on COVID-19 – 11 March 2020 [Internet]. 2020 [cited 2021 Nov 9]. Available from:—11-march-2020


[5] WHO. WHO Coronavirus (COVID-19) Dashboard [Internet]. 2021 [cited 2021 Nov 9]. Available from:


[6] Kim JH, Hotez P, Batista C, Ergonul O, Figueroa JP, Gilbert S, et al. Operation Warp Speed: implications for global vaccine security. Lancet Glob Health. 2021 Jul 1;9(7):e1017–21.


[7] FDA. FDA Takes Key Action in Fight Against COVID-19 By Issuing Emergency Use Authorization for First COVID-19 Vaccine [Internet]. FDA. FDA; 2020 [cited 2021 Nov 9]. Available from:


[8] FDA. Coronavirus (COVID-19) Update: FDA Authorizes Pfizer-BioNTech COVID-19 Vaccine for Emergency Use in Adolescents in Another Important Action in Fight Against Pandemic [Internet]. FDA; 2021 [cited 2021 Nov 9]. Available from:


[9] FDA. FDA Authorizes Pfizer-BioNTech COVID-19 Vaccine for Emergency Use in Children 5 through 11 Years of Age [Internet]. FDA; 2021 [cited 2021 Nov 9]. Available from:


[10] FDA. Learn More About COVID-19 Vaccines From the FDA. FDA [Internet]. 2021 Oct 29 [cited 2021 Nov 9]; Available from:


[11] Ritchie H, Mathieu E, Rodés-Guirao L, Appel C, Giattino C, Ortiz-Ospina E, et al. Coronavirus Pandemic (COVID-19). Our World Data [Internet]. 2020 Mar 5 [cited 2021 Nov 9]; Available from:


[12] Cavanaugh AM. Reduced Risk of Reinfection with SARS-CoV-2 After COVID-19 Vaccination — Kentucky, May–June 2021. MMWR Morb Mortal Wkly Rep [Internet]. 2021 [cited 2021 Nov 8];70. Available from:


[13] CDC. Frequently Asked Questions about COVID-19 Vaccination [Internet]. Centers for Disease Control and Prevention. 2021 [cited 2021 Nov 8]. Available from:


[14] Callaway E. COVID vaccine boosters: the most important questions. Nature. 2021 Aug 5;596(7871):178–80.


[15] Shrotri M, Navaratnam AMD, Nguyen V, Byrne T, Geismar C, Fragaszy E, et al. Spike-antibody waning after second dose of BNT162b2 or ChAdOx1. Lancet Lond Engl. 2021;398(10298):385–7.


[16] Boyarsky BJ, Werbel WA, Avery RK, Tobian AAR, Massie AB, Segev DL, et al. Antibody Response to 2-Dose SARS-CoV-2 mRNA Vaccine Series in Solid Organ Transplant Recipients. JAMA. 2021 Jun 1;325(21):2204–6.


[17] Abe KT, Hu Q, Mozafarihashjin M, Samson R, Manguiat K, Robinson A, et al. Neutralizing antibody responses to SARS-CoV-2 variants in vaccinated Ontario long-term care home residents and workers [Internet]. 2021 Aug [cited 2021 Nov 8] p. 2021.08.06.21261721. Available from:


[18] Fusco MD, Moran MM, Cane A, Curcio D, Khan F, Malhotra D, et al. Evaluation of COVID-19 vaccine breakthrough infections among immunocompromised patients fully vaccinated with BNT162b2 [Internet]. 2021 Oct [cited 2021 Nov 8] p. 2021.10.12.21264707. Available from:


[19] Barr B. What’s the difference between a COVID vaccine third dose and a booster shot? [Internet]. WGAL. 2021 [cited 2021 Nov 8]. Available from:


[20] FDA. Coronavirus (COVID-19) Update: FDA Authorizes Additional Vaccine Dose for Certain Immunocompromised Individuals [Internet]. 2021 [cited 2021 Nov 8]. Available from:


[21] FDA. FDA Authorizes Booster Dose of Pfizer-BioNTech COVID-19 Vaccine for Certain Populations [Internet]. 2021 [cited 2021 Nov 8]. Available from:


[22] FDA. Coronavirus (COVID-19) Update: FDA Takes Additional Actions on the Use of a Booster Dose for COVID-19 Vaccines [Internet]. 2021 [cited 2021 Nov 8]. Available from:


[23] CDC. COVID-19 Booster Shot [Internet]. Centers for Disease Control and Prevention. 2021 [cited 2021 Nov 8]. Available from:


[24] WHO. Statement of the WHO Global Advisory Committee on Vaccine Safety (GACVS) COVID-19 subcommittee on reports of Guillain-Barré Syndrome (GBS) following adenovirus vector COVID-19 vaccines [Internet]. 2021 [cited 2021 Nov 8]. Available from:


[25] Gargano JW, Wallace M, Hadler SC, Langley G, Su JR, Oster ME, et al. Use of mRNA COVID-19 Vaccine After Reports of Myocarditis Among Vaccine Recipients: Update from the Advisory Committee on Immunization Practices — United States, June 2021. Morb Mortal Wkly Rep. 2021 Jul 9;70(27):977–82.




South of Global Health

DISCLAIMER: The views and opinions expressed in this blog are the author's, and do not reflect the views of the Global Health Institute or the American University of Beirut.


إخلاء المسؤولية: الآراء الواردة في هذا المقال هي آراء المؤلف ولا تعكس آراء معهد الصحة العالمي أو الجامعة الأمريكية في بيروت


South of Global Health is a blog of the Global Health Institute at the American University of Beirut which tackles the Global South’s most pressing health issues across multiple intersectional themes. The blog intends to serve as an outlet for health-related issues affecting and originating from the most vulnerable regions of the developing world.


South of Global Health has an open and ongoing Call for Blog Entries. It accepts pieces between 500 and 1000 words, and adopts footnotes in APA as its referencing style. Kindly submit all entries or queries to the Editor at or with the subject “South of Global Health Blog Entry”.