Will you need two COVID-19 shots? Many vaccines require multiple doses. Here’s why
Ongoing clinical trials for a future novel coronavirus vaccine suggest people might need two doses instead of one in order to get the pandemic under control.
There are 23 vaccine candidates undergoing Phase 1 trials and 14 in Phase 2, with another nine options in Phase 3 trials being administered in thousands of volunteers: the last step before being approved and manufactured in mass, according to a New York Times vaccine tracker.
And the majority of these candidates have been given in more than one dose, meaning the real thing will likely be administered the same way.
But this doesn’t mean the future vaccine isn’t as effective as others, experts say. In fact, many vaccines given today require two or more doses to work including those for chicken pox, HPV, Hepatitis A and flu viruses.
Moderna, the Massachusetts-based biotech company behind a well-known Phase 3 coronavirus vaccine candidate, has been giving volunteers two doses with several weeks in between. The vaccine works by releasing pieces of the virus’s genetic material, or RNA, into human cells that later make proteins capable of mimicking the coronavirus, according to the company.
Eventually, the immune system recognizes the virus and creates antibodies against it to prevent infection. The technology has never been licensed for any disease, making this the first if given the green light from health officials.
The trials are designed to learn that “sweet spot that produces the best immune response in the shortest amount of time,” Kelly Moore, the associate director for immunization education with the Immunization Action Coalition, told KTVU.
And that sweet spot for the coronavirus likely involves two doses. Here are some reasons why:
Immunity can wear off over time
The Centers for Disease Control and Prevention says that for some vaccines, one dose is simply not enough to give people “as much immunity as possible.” So, two doses are required to “build more complete immunity.”
The vaccine against meningitis, an infection that causes inflammation in the brain and spinal cord membranes, is one example.
Some vaccines need “boosters” several years after initial doses because immunity for certain illnesses naturally wears off, according to the CDC, such as those for the flu.
The DTaP vaccine that protects against diphtheria, tetanus and pertussis is administered in four doses, with the first booster injected at about 5 years old when immunity levels have dropped, the CDC said.
“The first time you are presented with a pathogen, you generate an immune response,” Dr. Pritish Tosh, an infectious diseases physician and researcher at the Mayo Clinic, told The Huffington Post in 2015. “But then if you are presented with that same pathogen again a few years later, your immune system is able to produce a more specific and longer-lasting response in general” with a second dose of a vaccine.
Just because some vaccines may not be enough the first time around doesn’t mean that “naturally acquired immunity” is an easier way out. The CDC says that natural infections for some illnesses “can cause severe complications and be deadly…. it is impossible to predict who will get serious infections that may lead to hospitalization.”
One dose may not be enough for ‘best immune response’
Like Moore said, more than one dose might be required so that people can develop the best immune response over time.
One dose of the MMR vaccine against the measles, mumps and rubella usually is not enough to produce a sufficient amount of antibodies needed to ensure everyone’s protection, the CDC said.
And in other cases, some viruses can change enough that a vaccine administered last year is no longer effective this year. Flu viruses tend to behave this way with their rapid mutation rate.
“Sometimes, if you take a large group of people with one vaccination you might expect 90 percent [to be protected],” Tosh told the Huffington Post. “But if you give a second dose, you may get up to 98 percent… what is probably a more straightforward strategy is just giving two doses to insure you have that high level of protection.”
This story was originally published September 1, 2020 at 3:20 PM with the headline "Will you need two COVID-19 shots? Many vaccines require multiple doses. Here’s why."
