This evidence review was compiled by members of the CITF Secretariat with the input from experts affiliated with the CITF and does not necessarily reflect the views of all CITF members.

By Mariana Bego

COVID-19 vaccines are effective at preventing hospitalizations and emergency department visits caused by the Delta variant, according to several recent publications. The data also indicate that Moderna’s vaccine is significantly more effective against Delta than Pfizer-BioNTech or Johnson & Johnson’s Janssen.

Real-world data show that vaccines remain highly effective at reducing COVID-19 related hospitalizations and emergency department (ED) visits, even in the presence of the circulating COVID-19 variants. Vaccination is therefore highly important to ease the burden on the healthcare system.

Recent key reports:

In a recent correspondence to The New England Journal of Medicine, a group in Scotland evaluated whether vaccination prevented viral transmission from healthcare workers to members of their households. They evaluated data from over 190,000 household members of almost 150,000 healthcare workers between March 2020 and March 2021.1

  • Cases of COVID-19 were less common among household members of vaccinated healthcare workers compared to unvaccinated (transmission rates were 9% before the first dose, 6% within the first 14 days after the first dose, and 3% after the second dose). This suggests that vaccination may reduce transmission.
  • Vaccination was also associated with a reduction in the number of COVID-19–related hospitalizations in healthcare workers beginning 14 days after the first dose.

The US Centers for Disease Control and Prevention (CDC) recently released two reports regarding vaccine effectiveness against hospitalization; two other reports were also made available in early September; all four reports are summarized below.

The first CDC report included over 40,000 cases of adults aged >50 years during the months of January to June 2021.2

  • mRNA vaccines (Pfizer or Moderna) were 89% effective against laboratory-confirmed SARS-CoV-2 infection leading to hospitalization, whereas the effectiveness of Johnson & Johnson’s vaccine dropped to 68% for the same effectiveness criteria.

The second CDC report included over 30,000 cases during June, July, and August 2021, when the Delta variant became the predominant strain3, in adults aged ≥18 years.

  • Unvaccinated individuals who contracted COVID-19 are 5 to 7 times more likely to need ED care or hospitalization when compared to vaccine recipients.
  • Mimicking the previous report, Moderna was 95% effective at preventing hospitalizations among adults ages 18 and older, followed by Pfizer and Johnson & Johnson/Jansen (at 80% and 60%, respectively).
  • Vaccine effectiveness was lower for those aged 75 years and above.

Another report in The Lancet Regional Health – Americas, stated that “The need for emergency care/hospitalization due to breakthrough COVID-19 is an exceedingly rare event in fully vaccinated patients.” That study followed over 10,000 COVID-19 emergency care visits from December 2020 to April 2021 in Michigan, US.4

  • Like in other studies, vaccine effectiveness was lower among the elderly.
  • In the fully vaccinated group (129 individuals), the authors reported eight deaths and six intubations that occurred in patients over the age of 65. Comparatively, in the unvaccinated group (over 10,000 individuals), patients as young as 21 died while hospitalized and patients as young as 19 required mechanical ventilation.

Finally, a report in The Lancet Infectious Diseases conducted a systematic review of patients admitted to the hospital with SARS-CoV-2 (confirmed by a positive PCR test at the time of admission) in Yale, England, between March and July, 2021.5

  • Of the patients studied, 54 were fully vaccinated prior to infection and therefore were deemed to have a breakthrough SARS-CoV-2 infection. When evaluating illness severity among these 54 patients, authors noted that 25 (46%) were asymptomatic (admitted to the hospital for a non-COVID-19-related diagnosis but with an incidental positive PCR test for SARS-CoV-2), four (7%) had mild disease, 11 (20%) had moderate disease, and 14 (26%) had severe or critical illness.
  • Among the 14 with severe or critical illness, the median age was 80 years (range 76 to 85), 13 had received the Pfizer vaccine, four required intensive care, one required mechanical ventilation, and three died.
  • Pre-existing comorbidities in the 14 patients with severe or critical illness included cardiovascular disease (12/14), overweight (9/14), lung disease (7/14), type 2 diabetes (7/14), malignancy (4/14), and use of an immunosuppressive agent (4/14).


1- Shah ASV, Gribben C, Bishop J, Hanlon P, Caldwell D, Wood R, Reid M, McMenamin J, Goldberg D, Stockton D, Hutchinson S, Robertson C, McKeigue PM, Colhoun HM, McAllister DA. Effect of Vaccination on Transmission of SARS-CoV-2. N Engl J Med. 08 Sept 2021. DOI: 10.1056/NEJMc2106757

2- Thompson MG, Stenehjem E, Grannis S, et al. Effectiveness of Covid-19 vaccines in ambulatory and inpatient care settings. N Engl J Med. 08 Sept 2021. DOI: 10.1056/NEJMoa2110362.

3- Grannis SJ, Rowley EA, Ong TC, Stenehjem E, Klein N, DeSilva MB, Naleway AL, Natarajan K, Thompson MG, VISION Network. Interim Estimates of COVID-19 Vaccine Effectiveness Against COVID-19–Associated Emergency Department or Urgent Care Clinic Encounters and Hospitalizations Among Adults During SARS-CoV-2 B.1.617.2 (Delta) Variant Predominance — Nine States, June–August 2021. MMWR Morb Mortal Wkly Rep. 10 Sept 2021. DOI: 10.15585/mmwr.mm7037e2

4- Bahl A, Johnson S, Maine G, Hernandez Garcia M, Nimmgadda S, Qu L, Chen N. Vaccination reduces need for emergency care in breakthrough COVID-19 infections: A multicenter cohort study. Lancet Reg Health Am. 09 Sept 2021. DOI: 10.1016/j.lana.2021.100065

5- Juthani PV, Gupta A, Borges KA, et al. Hospitalisation among vaccine breakthrough COVID-19 infections. Lancet Infect Dis. 07 Sept 2021. DOI: 10.1016/S1473-3099(21)00558-2