How Effective Is the COVID Vaccine?

Both the COVID-19 vaccine developed by Pfizer and BioNTech, and the one developed by Moderna and the National Institute of Allergy and Infectious Diseases, were found to be around 94 to 95 percent effective in clinical trial settings.

However, further studies will be done to determine how well the vaccines work in real-life settings, the U.S. Centers for Disease Control and Prevention (CDC) has said.

According to a briefing by the Food and Drug Administration (FDA) on the Moderna vaccine, which is under final review on Thursday for emergency us authorization, its "efficacy in preventing confirmed COVID-19 occurring at least 14 days after the second dose of vaccine was 94.5 percent," in a trial involving approximately 30,400 participants.

In an FDA briefing on Pfizer's vaccine, which was approved for emergency use last week, "efficacy in preventing confirmed COVID-19 occurring at least 7 days after the second dose of vaccine was 95.0 percent," in a trial involving about 44,000 people.

The CDC advised last week: "After FDA approves a vaccine or authorizes a vaccine for emergency use, it continues to be studied to determine how well it works under real-world conditions. CDC and other federal partners will be assessing COVID-19 vaccine effectiveness under real-world conditions."

The federal agencies working with the CDC on these latest assessments include the FDA, the Centers for Medicare and Medicaid Services, the Department of Defense, the Indian Health Service and the Veteran's Health Administration.

"These real-world assessments will compare groups of people who do and don't get vaccinated and people who do and don't get COVID-19 to assess how well COVID-19 vaccines are working to protect people," the CDC explained.

Assessments will also be made on how well the vaccines protect people against severe COVID-19 illness, as well as against less severe forms of COVID-19.

The assessments will be done across diverse groups of people, including healthcare personnel, essential workers, older adults, those living in nursing homes, people with underlying medical conditions, different racial and ethnic minority groups as well as tribal nations, the CDC noted.

How COVID-19 vaccines are tested

Experts are working on various types of real-world assessments using different methods including the following, as outlined by the CDC:

Case-control studies

"These assessments will include cases (people who have the virus that causes COVID-19) and controls (people who do not have the virus that causes COVID-19). The people who agree to participate in a case-control assessment will provide information on whether they received a COVID-19 vaccine or not.

Experts will look to see if the cases were less likely to have received the vaccine than controls, which would help show that the vaccine is working.

Test-negative design

"This is a special type of case-control study. These assessments will enroll people who are seeking medical care for symptoms that could be due to COVID-19. Experts will then compare the COVID-19 vaccination status of those who test positive (meaning they have COVID-19) to those who test negative (meaning they do not have COVID-19).

Cohort studies

"These assessments will follow people who have and haven't had a COVID-19 vaccine for several months to see if getting vaccinated protects them from getting the disease. This can be done in real time (prospectively) or by looking back in time (retrospectively) using data that were already collected, such as information in participants' electronic health records (medical records).

Screening method

"These assessments look at vaccination coverage among a group of cases (for example, cases detected through ongoing COVID-19 surveillance) and compares it with vaccination coverage among the overall population where those cases come from (for example people from the same state). By comparing coverage among these two groups, researchers can get an early estimate of whether a vaccine is working as expected.

Ecologic analyses

"These assessments look at groups of people – such as those in different geographic locations or at different times – and find out how many people were vaccinated and how many people were diagnosed with COVID-19. These analyses may be hard to interpret since the number of COVID-19 illnesses has changed rapidly over time and in different places," the CDC noted.

UCLA doctor receives COVID-19 vaccine December 2020
A University of California, Los Angeles (UCLA) doctor being inoculated with the COVID-19 vaccine at the Ronald Reagan UCLA Medical Center in Westwood, California on December 16. The two COVID-19 vaccines developed by Pfizer and Moderna were found to be 94 to 95 percent effective in clinical trial settings. Brian Van Der Brug/Pool/AFP via Getty Images

The wider picture

The novel coronavirus has infected more than 74.2 million people, including over 16.9 million in the U.S., since it was first reported in Wuhan, China.

More than 1.6 million people have died worldwide and over 42 million have recovered as of Thursday, according to John Hopkins University.

The graphic below, provided by Statista, shows the percentage of adults in the U.S. who would or would not get a COVID-19 vaccine.

COVID vaccine hesitancy in U.S.