Most COVID-19 vaccines are given as two shots, administered several weeks apart.
Protection does not start when the needle hits your arm.
It takes some days after each shot for the body to mount its own immune response to the novel coronavirus and prevent disease.
Experts don't know exactly how protected from infection people are after their first shot, but there are signs of slight — but not full — protection after a couple weeks.
Only weeks after a second shot should people feel secure that they have a very low chance of getting COVID-19. Even then, their risk is not zero.
As 2021 begins, millions of people around the world are starting to get COVID-19 vaccines.
Shots that have been approved so far (Pfizer and Moderna in the US; Pfizer and AstraZeneca in the UK) have proved highly effective, providing up to 95% protection from the disease caused by the novel coronavirus.
However, vaccine immunity doesn't kick in immediately. Each of these vaccines requires two shots, administered several weeks apart, in order to provide people with a robust, long-lasting form of protection against the virus.
Although there is some evidence a first dose can start bolstering the body's defenses against the novel coronavirus, it's only after the second shot that your risk of infection can plummet to as little as 5%.
As Pfizer CEO Albert Bourla said in December, it would be a "big mistake" to rely on just one shot of his company's vaccine to keep you safe from disease. While one shot could help begin to control the pandemic, Bourla told reporters, "with two, you almost double the protection."
Here's what we know - and don't know yet - about when and how these new shots protect people from infections.
Yes, you can get COVID-19 after the first shot of a two-dose vaccine
Already, there have been several instances of people who have gotten their first shot subsequently getting infected with the novel coronavirus.
One nurse in California got COVID-19 six days after his first shot, and another emergency-room doctor in Georgia came down with COVID-19 nine days after his first dose of Pfizer's vaccine.
"This was just dumb luck," Josh Mugele, the ER doctor, told Insider's Aria Bendix. "I happened to be exposed within a few days of getting the vaccine, but this still is the best tool we have for fighting the virus."
Vaccinated people do get some form of protection from their first shot, which generally kicks in after about two weeks. But two shots have been shown to be more effective for stronger, longer-lasting immunity.
"The second dose gives you ten times higher immune response than the first dose," Moncef Slaoui, the US's chief science advisor for the vaccine rollout, said on CBS Sunday.
Vaccines train the body to fight the coronavirus, and that takes time
Pfizer and Moderna have created mRNA vaccines, which include pieces of genetic material that teach the body to recognize and attack something called "the spike protein" - a unique feature of the novel coronavirus that latches onto human cells.
It works fast, but not instantaneously.
"Your body is going to take a little time to build up a robust immune response that is going to provide you protection, and unfortunately, that protection doesn't come as soon as the needle breaks your skin," Dr. Wesley Willeford, Medical Director of Disease Control at the Jefferson County health department in Alabama, recently told WBRC.
"Once you're vaccinated, and get both doses, you still need to wait two to four weeks before really beginning to think about re-engaging in a lot of activities."
Vaccines have been designed to be most effective after a 2nd 'booster' shot
When COVID-19 vaccines were tested in tens of thousands of volunteers around the world, scientists relied on a two-dose regimen. Any claims you might hear that vaccines are up to 95% effective at preventing COVID-19 infections are based only on trial results using two shots, given several weeks apart.
Before vaccine efficacy (that is, how well shots work at preventing symptomatic infections) was measured, patients waited one to two weeks after their second shot, to make sure their immunizations had ample time to take effect. Even then, shots were not perfect at preventing every single infection.
Pfizer-BioNTech's vaccine is up to 95% effective. It consists of two shots administered 21 days apart.
According to the US Food and Drug Administration, Pfizer's first shot can provide up to 52% protection. But it's not until a week after the second shot that the chance of falling ill from the coronavirus plummets to just 5%.
That means if someone receives Pfizer's two shots exactly 21 days apart, they should not expect to be fully protected until roughly a month (28 days) after their first dose.
Moderna's results look similar. According to trial results, the first shot may provide good protection, around the order of 90%, after 14 days. But it's not clear exactly how long that protection lasts, or how good it really is, because almost everyone in the trials got a second booster shot 28 days later.
What we do know is that two shots protects people almost entirely from the most severe, potentially deadly infections.
The coronavirus vaccine produced by AstraZeneca and Oxford University, which uses a different kind of viral vector technology, was authorized for use in the UK on December 30. It also consists of two doses administered up to 12 weeks apart. It was shown to be about 62% effective with two full doses, but in a surprise find, immunity from the vaccine soared as high as 90% when patients were accidentally administered a half-dose first shot followed by a full dose booster.
It's possible that different doses, combinations, or timings of any these vaccines could produce better results, or that distributing smaller doses more widely across the population would allow us to safely and effectively vaccinate more people more quickly. We just don't know yet, because none of this has been properly studied.
Scientists in the UK are now conducting a so-called "mix and match" trial, aiming to discover whether using one shot of Pfizer's mRNA vaccine, along with one shot of AstraZeneca's viral vector vaccine, might provide people with more complete protection than a single brand alone.
For now, the FDA maintains that "without appropriate data supporting such changes in vaccine administration, we run a significant risk of placing public health at risk" trying any of these untested strategies out on the general public.
Delaying the second shot could be a gamble, but it could also save some lives
With new, possibly more contagious variants of the virus spreading fast around the world now, regulators in the UK have decided to try to get as many people as possible inoculated with one shot, giving them some form of protection in the hopes of quickly hobbling the pandemic.
Experts say this is still a risky, untested strategy.
"We want to act quickly, but we need the trials," Dr. Paul Offit, a professor of vaccinology at the University of Pennsylvania and Director of the Vaccine Education Center, told Insider.
Not everyone shares his caution.
Dr. Stanley Plotkin, a professor emeritus at the University of Pennsylvania, recently wrote a letter to the director of the Centers for Disease Control and Prevention, urging that during a pandemic raging out of control across the US, the one shot strategy could be beneficial.
"It makes sense to try to protect as many people as possible," he told Insider. "One dose could give a lot of protection ... in the worst case, it will at least reduce the severity of the disease."
Do you have a story to share about getting the vaccine? Contact public health reporter Hilary Brueck by email.
Megan Hernbroth contributed reporting.
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