But Dr. Chris Gill, an infectious disease specialist at Boston University, points out that a single injection of Pfizer's vaccine may be even more effective than this estimate suggests. Looking at data from a smaller window between the time the first injection should have started working and before the second injection kicked in, Gill says the Pfizer-BioNTech vaccine may have an efficacy rate as high as 80 or 90% with just a single dose.

Moderna actually collected data from people who only received one dose of its vaccine, Gill says. Some 2,000 participants in Moderna’s phase three clinical trial received just a single injection of either a placebo or the vaccine. In that population, the efficacy of the single vaccine dose was roughly 80 to 90%.

“[Moderna] was not shy about showing that a single dose was so effective, and they do the math right," Gill says. “After 14 days, the [single dose] vaccine is remarkably effective.” //

Dr. Benjamin Linas, an epidemiologist also at Boston University, is still mulling over the question. There’s still a lot of information that is yet to be revealed, he says. For example, is it less effective to receive the second dose of the vaccine a few months later than recommended schedule?

“Probably not, but no one knows,” Linas says.

And nobody knows how long the protection from a single dose will last. Of course, nobody knows how long the protection from two doses of the vaccines will last, either. Beyond the roughly two-month period of the clinical trials, those studies haven't been done yet.

There is another concern throwing its shadow over the proposal to vaccinate more people now with a single dose: How to convince millions of people to show up for a second dose at an unspecified point in the future. It's hard enough to get people to arrive at pre-scheduled appointments for a second shot a month later, Linas says, let alone an unknown date based on an unknown supply of vaccines.

“If we gave all the vaccine now and back fill the second doses later, do we really have the logistical support to do that without entering chaos?” Linas says. “It makes me a little nervous.”

Single doses of the Pfizer and Moderna vaccines are more than 92 percent effective in preventing COVID-19 illness after two weeks, Canadian researchers are saying.

The FDA’s own data show that a single shot of the BioNTech-Pfizer vaccine is 92.6 percent effective after two weeks, and a single Moderna jab is 92.1 percent effective, the researchers note in the New England Journal of Medicine. //

“There may be uncertainty about the duration of protection with a single dose,” the researchers said.

• Millions of people worldwide have had one shot of two-dose COVID-19 vaccines.
• Pfizer and Moderna vaccines are likely 80% effective against symptomatic COVID-19 after one dose.
• A single AstraZeneca shot is probably at least 70% effective at preventing symptomatic COVID-19. //

Stephen Evans, a professor of medical statistics at the London School of Hygiene & Tropical Medicine and a former drug-safety committee member at the European Medicines Agency, helped Insider break down the data. //

Percentage efficacy for vaccines refers to the proportion of people that get full protection after a vaccine. With 80% efficacy, 80% of people have full protection, and 20% don't.

For those who get full protection the first time around, the second shot improves the quality of the immune response and its durability.

For the people who don't get full protection with the first shot, some will get full protection after the second dose. Some people won't ever get full protection from a vaccine because their immune system doesn't respond at all. //

A UK study found Pfizer or AstraZeneca's vaccine cut COVID-19 infections with symptoms by 72% after one dose, and protection probably held up for 10 weeks. Protection from Pfizer's vaccine rose to 90% after two doses. The study hasn't been peer-reviewed.
A US study of essential workers found that a single dose of Pfizer of Moderna's COVID-19 vaccines were 80% effective against all coronavirus infections from 14 days.
A Scottish study found that a single dose of Pfizer's vaccine was 91% effective against hospitalization at 28 to 34 days following vaccination. One dose of AstraZeneca's vaccine was 88% effective against hospital admissions after the same time period.
A UK study found that a single dose of either Pfizer or AstraZeneca's vaccine cut spread of symptomatic COVID-19 within a household by up to 50%.
A South Korean study found one dose of Pfizer's vaccine was 89.7% effective at preventing COVID-19 in South Koreans aged over 60, at least two weeks after vaccination. AstraZeneca's vaccine was 86% effective at preventing COVID-19 after one dose. The severity of illness that the shots protected against was unclear — generally they're more effective at preventing COVID-19 infections that caused hospitalization or death.
An English study found that a single dose of either Pfizer or AstraZeneca's vaccine was about 80% effective at preventing hospitalization in people over 70-years-old. Protection lasted for at least 6 weeks, including against the Alpha variant first identified in the UK.

Pretend it didn't happen – expert advice on how to behave after receiving a single dose of any of the Covid-19 vaccines. //

When the immune system first encounters a vaccine, it activates two important types of white blood cell. First up are the plasma B cells, which primarily focus on making antibodies. Unfortunately, this cell type is short-lived, so although your body might be swimming in antibodies within just a few weeks, without the second shot this is often followed by a rapid decline.

Then there are the T cells, each of which is specifically tailored to identify a particular pathogen and kill it. Some of these, memory T cells, are able to linger in the body for decades until they stumble upon their target – meaning immunity from vaccines or infections can sometimes last a lifetime. But crucially, you usually won't have many of this cell type until the second meeting.

The booster dose is a way of re-exposing the body to the antigens – the molecules on pathogens that trigger the immune system – to initiate part two of the response. "You've kicked in all this fancy stuff," says Altmann. "So, once you've had your boost you'll have a higher frequency of memory T cells and ditto to some extent for the size of the pool of memory B cells you'll have. They'll also be making higher quality antibodies."

On second exposure to the same vaccine or pathogen, the B cells that remain from before are able to rapidly divide and create a menacing throng of descendants, leading to a second spike in the amount of antibodies circulating.

The second dose also initiates the process of "B cell maturation", which involves selecting the immature ones with the best receptors for binding to a particular pathogen. This happens while they're still in the bone marrow – where white blood cells are made – and afterwards they travel to the spleen to finish developing. This means B cells are not only more numerous afterwards, but the antibodies they produce are better targeted.

Recent studies show vaccines for mumps, pertussis, meningococcal disease, and yellow fever also lose their effectiveness faster than official immunization recommendations suggest. Vaccines have been a crucial public health tool for decades, so it may seem strange that their durability isn't well understood. But vaccines are approved and come to market years before it's clear how long protection lasts. Later, fading protection can go unnoticed because a vaccine in wide use has largely eliminated transmission of the microbes it protects against, making "breakthrough" infections rare. Even if viruses or bacteria are still in circulation, people vaccinated against them will sometimes receive natural boosting of their immunity. And declining vaccine immunity is not an all-or-nothing phenomenon: A breakthrough infection often leads to much less severe symptoms of the disease.

Researchers are ramping up efforts to figure out why some vaccines protect for mere weeks but others work for life. "We simply don't know what the rules are to inducing long-lasting immunity," says Plotkin, who began to research vaccines in 1957. "For years, we were making vaccines without a really deep knowledge of immunology. Everything of course depends on immunologic memory, and we have not systematically measured it."

“Alexandra Bowie of Regeneron told news aggregation platform Heavy in an October email that REGN-COV2 was not made with human embryonic stem cells.

“This particular discovery program (REGN-COV2) did not involve human stem cells or ESCs,” Bowie wrote. “