In June, the FDA
issued guidelines for the approval of COVID-19 vaccines. In those guidelines, the agency described what safety evaluations a vaccine would need to pass and noted that a vaccine should show at least 50% efficacy in clinical trials, meaning that under perfect conditions, people who get the vaccine would be at least half as less likely to be infected with the coronavirus compared with unvaccinated people.
"This is a common guideline for flu vaccine testing from the FDA and has been shown in the past to be an effective marker for determining vaccine efficacy," Salem said.
The efficacy of "50% is lower protection than we see with some other vaccines, but reducing the case burden by 50% is still significant," Dr. Sarah George, an associate professor of infectious diseases and immunology at Saint Louis University, told Live Science in an email. "Remember, every case you stop from happening means you’ve also interrupted the transmission cycle, so [the 50% cutoff is] appropriate." With fewer people to infect, the virus cannot spread through a community as quickly; combined with other disease mitigation measures like social distancing and mask wearing, vaccines can drastically reduce the chances of an infected person passing on the virus.
Scientists can begin to assess a vaccine's efficacy in Phase 2 and Phase 3 clinical trials by monitoring how the body responds to the inoculation,
according to the U.S. Centers for Disease Control and Prevention (CDC). Ideally, the
immune system would make neutralizing antibodies that target the SARS-CoV-2 virus and thus protect the vaccinated person, if they are ever exposed to the pathogen.
Phase 3 trials, which include hundreds to tens of thousands of volunteers, can begin to reveal differences in infection rates between vaccinated and unvaccinated people — but those trends become clearer the more people who are included in the trial, and the longer the trial lasts.
https://www.livescience.com/worst-epidemics-and-pandemics-in-history.html
Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in April that
a promising COVID-19 vaccine that prompts a strong immune response in Phase 2 trials could potentially be approved for emergency use without a full Phase 3 trial,
Live Science previously reported.
Phase 3 trials for vaccines typically last several years,
according to the CDC, but given the pressing need, a coronavirus vaccine could potentially be granted emergency approval in less time, Fauci said. The trial process can also be sped up through combined trials, where several trial phases are run simultaneously.
While the idea of condensed or combined trials may seem risky, "that doesn't mean they're cutting off the follow-up of these individuals," Bottazzi said. "Importantly, beyond Phase 3 there's a clear strategy and guidance [from the FDA] on post-market surveillance," which involves monitoring people who receive an approved vaccine for infections and side effects that may not appear in clinical trials, she said.
But the fact remains that "antibody responses alone cannot be used to determine effectiveness of a vaccine and its ability to prevent infection," or to reduce the severity of infection, Salem noted. Those metrics can be calculated only through large, often lengthy, Phase 3 trials and thorough post-market surveillance.
Related: 14 Coronavirus myths busted by science
Well-designed clinical trials will tell us if a COVID-19 vaccine is safe and effective, but they cannot answer one crucial question: How long will the vaccine protect a person from the virus?
Immunity to common coronaviruses, which cause symptoms of the common cold, dwindles within about one to three years,
Live Science previously reported. Similarly, past studies of SARS and the coronavirus called Middle East respiratory syndrome (MERS) hinted that people may remain immune to the coronaviruses for at least two or three years after their initial infection. A COVID-19 vaccine should prompt a similar immune response to the natural infection, and therefore, immunity granted by the vaccine may also wane through time.
"We do not know how long that protection will last, and we probably need to look at other formulations to improve on that front" after an initial vaccine gets approved, Bottazzi said. "That's why we most likely will have first, second, third generations of vaccines."
In addition to designing vaccines that grant long-lasting immunity, developers may need to come up with different formulations for people of different demographics. For example, "separate studies in children will have to be done once we have one [vaccine] approved for adults," George said.
The first COVID-19 vaccine may not work for everyone, or may offer only partial immunity, "I think it's a good start," Bottazzi said. In the short-term, a moderately effective vaccine would prevent a subset of people from catching COVID-19 and slow viral transmission in the wider community by limiting the number of potential infections. Vaccinated people who still caught COVID-19 would likely contract a less severe illness than unvaccinated people, reducing the burden on the health care system and the mortality rate, overall, she said. And given that the long-term impacts of the infection remain unknown, a vaccine may protect people from future health complications, as well, she added.
"An effective vaccine against SAR-CoV-2 remains our greatest chance of getting COVID-19 under control and having life go back to normal," Salem said.
https://www.livescience.com/first-coronavirus-vaccine-safety.html
