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Are COVID-19 vaccinations effective?

Does vaccination protect me from COVID-19?

Yes. When you are vaccinated, your immune system learns how to combat the invading coronavirus, thus protecting your body from COVID-19.

It is very important to build immunity against the coronavirus through vaccination since it prevents not only infection, but also serious illness and even death.

Has any study been conducted on COVID-19 vaccination effectiveness in South Korea?

According to the study on vaccine effectiveness among vaccinated and unvaccinated individuals aged 60 or older from February 26 through April 26, 2021 in South Korea, a single dose of COVID-19 vaccine was on average 86.6% effective in preventing infections after two weeks ― the AstraZeneca shot was found to be 86.0% effective and the Pfizer vaccine 89.7% effective.

[Analysis Method]
  • The effectiveness of a single dose of vaccine was calculated using the Cox proportional hazards model* to correct the influence of follow-up period, gender, age, and location from February 26 through April 26, 2021.**
  • * Cox proportional hazard model: A survival analysis method that reflects the follow-up period and adjusts variables to compare and analyze the attack rate of disease and other risk factor levels among two groups with different characteristics
  • ** Assigning a hazard ratio to each variable through the Cox proportional hazards model: (1 – hazard ratio) × 100%

Effectiveness of COVID-19 Vaccines Among People Aged 60 or Older (as of April 26, 2021 at 24:00, Unit: number of person)

Effectiveness of COVID-19 Vaccines Among People Aged 60 or Older (as of April 26, 2021 at 24:00, Unit: person)
Vaccination History Number of Subjects (person) Follow-Up Period (day) No. of Confirmed Cases Adjust Hazard Ratio*
(95% confidence interval)
Number of Subjects (person)Vaccine Effectiveness**
(95% confidence interval)
Total (4,416,635) Unvaccinated 3,063,010 104,452,532 1,237
14 days or more after vaccination 521,133 14,568,048 29 0.134
(0.09-0.20)
86.6%
(80.3-91.0)
AstraZeneca Plc (801,119) Unvaccinated 310,713 10,890,330 161 1 -
14 days or more after vaccination 320,332 10,660,328 25 0.140 (0.09-0.22) 86.0% (78.2-91.0)
Pfizer (3,615,516) Unvaccinated 2,752,297 93,562,202 1,076 1 -
14 days or more after vaccination 200,801 3,907,720 4 0.103
(0.04-0.28)
89.7%
(72.4-96.1)

*Hazard ratio: A ratio of infection risks of vaccinated and unvaccinated populations estimated through the Cox proportional risk model by standardizing the influence of gender, age, and location

**Vaccine effectiveness: (1 – hazard ratio) × 100%

Both vaccines ― AstraZeneca and Pfizer ― are confirmed to have a high effectiveness against infection for a certain period of time with a single dose of vaccination. However, full vaccination (second dose) within the recommended period is necessary for full protection.

What does vaccine efficacy mean?

Vaccine efficacy expressed in a percentage indicates the reduction in the risk of COVID-19 infection among the vaccinated group compared to the unvaccinated control group.

The most commonly used measure of vaccine efficacy is the proportionate reduction in AR between ARU (control group) and ARV. The vaccine efficacy can be expressed as (ARU-ARV/ARU)×100 or (1-RR)×100 to estimate the reduced RR from vaccination. 1)
* AR: attack rate, RR: relative risk
ARU: attack rate in unvaccinated population
ARV: attack rate in vaccinated population

1) Considerations in COVID-19 Vaccines Development, June 15, 2020. National Institute of Food and Drug Safety Evaluation, Ministry of Food and Drug Safety (MFDS)

What are the criteria necessary for COVID-19 vaccine efficacy?

South Korea defines the vaccine efficacy threshold as a point estimate of at least 50% with the lower bound of 95% confidence set above 30%. 2)

The World Health Organization (WHO)3)and the U.S. FDA4)also set the standards ― a point estimate of at least 50% with a 95% confidence interval lower limit of 30% or higher ― same as South Korea. 2) onsiderations in COVID-19 Vaccines Development, June 15, 2020. National Institute of Food and Drug Safety Evaluation, MFDS
3) Considerations for Evaluation of COVID-19 Vaccines, Points to Consider for Manufacturers of COVID19 Vaccines, Version 24 September 2020, WHO
4) Development and Licensure of Vaccines to Prevent COVID-19, Guidance for Industry, June 2020, U.S. FDA

What are the clinical trial results on the efficacy of COVID-19 vaccines?

According to the clinical trial data filed for approval review by the Ministry of Food and Drug Safety, the AstraZeneca COVID-19 vaccine was 62% effective at preventing the disease.5)
* Two clinical trials were conducted in the U.K. (Phases 2 and 3) and Brazil (Phase 3) on 8,895 people who did not have previous COVID-19 infection and were aged 18 or older, among whom 4,440 were in the vaccinated group and 4,455 in the control group (comparison group). There were 27 cases of COVID-19 in the vaccine group, and 71 cases in the control group.

According to the clinical trial data filed for approval review by the Ministry of Food and Drug Safety, the efficacy of the Pfizer COVID-19 vaccine was evaluated as 95%.6)
* Multinational clinical trials were conducted in countries including the U.S. on 36,523 participants (18,198 in the vaccinated group and 18,325 in the control group) who were 16 years and older and had not been previously infected by SARS-CoV-2 with 8 and 162 cases in each group, respectively.

According to the clinical trial data submitted for approval review by MFDS, the Janssen COVID-19 vaccine was 66.9% effective 14 days after vaccination, with 116 and 348 cases occurring in the vaccinated and control groups, respectively; and 66.1% effective 28 days after vaccination, with 66 cases in the vaccinated group and 193 in the control group.7)
* Results from a multinational clinical trial conducted in the U.S. and other countries in 39,321 participants aged 18 and older without evidence of prior SARS-CoV-2 infection (19,630 in the vaccinated group and 19,691 in the control group)

According to the clinical trial data submitted for approval review by MFDS, the efficacy of the Moderna vaccine was found to be 94.1%.8)
* During a clinical trial conducted in the U.S., there were 11 cases (out of 14,134 participants) in the vaccinated group and 185 cases in the control group (out of 14,073 participants) after two doses of either Moderna COVID-19 vaccine or placebo.
5) MFDS Grants Marketing Authorization for Korea AstraZeneca’s COVID-19 Vaccine, MDFS Press Release, February 10, 2021
6) MFDS Grants Marketing Authorization for Pfizer Korea COVID-19 Vaccine, MDFS Press Release, March 5, 2021
7) MFDS Grants Marketing Authorization for Janssen COVID-19 Vaccine, MDFS Press Release, April 7, 2021
8) MFDS Grants Marketing Authorization for Moderna COVID-19 Vaccine, MDFS Press Release, May 21, 2021

According to the clinical trials of the Pfizer, Moderna, and AstraZeneca vaccines, their efficacy as a whole exceeds the 50% threshold set by WHO, and thus they are expected to be effective in preventing and containing the spread of COVID-19.

How long does immunity last after COVID-19 vaccination?

Since COVID-19 is a relatively new disease and the vaccines were developed fairly recently, it is not yet known how long the vaccine’s protective effect will last.

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