The race is well under way to vaccinate the global population against coronavirus, delivering in most cases two doses to billions of people around the world.
But already drug makers and governments are considering the likelihood that third doses or "booster shots" will be needed.
Here we look at why that is, what they could consist of and whether there are any potential health risks.
Do I need a Covid-19 booster shot?
Although there is limited data available so far, the protection from having two doses is likely to wane over time, so a booster might be needed to ensure the immune response persists.
Many vaccines against long-established diseases, such as hepatitis A, require booster shots if protection is to be sustained.
As reported in The National on Friday, Dr Nawal Al Kaabi, chairwoman of the National Covid-19 Clinical Management Committee, suggested that immunity from Covid-19 conferred by China's Sinopharm vaccine is likely to last for four to six months, so there was "a possibility" a booster would be needed.
A third vaccine dose – even if it is of the same vaccine given previously – is also being considered as a strategy to cope with emerging variants, which some of the existing vaccines are not as effective against.
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Kathi Carlsten, a Los Angeles Unified School District nurse, applies a dose of the Moderna vaccine to a LAUSD employee as LAUSD's first vaccination site opened at the Roybal Learning Center, in Los Angeles. Superintendent Austin Beutner was there in the morning to meet with the vaccination team and the first district employees to receive their vaccines at this site. AP Photo -
Medical transporter Adrian Parrilla transports a patient into a COVID-19 unit at Mission Hospital in Mission Viejo, Calif. AP Photo -
A health worker receives a dose of the Pfizer-BioNTech vaccine against COVID-19 at the Versalles Clinic, in Cali, Colombia, amid the novel coronavirus pandemic. AFP -
A commuter wears a faces shield and mask across the street from a train station amid the COVID-19 pandemic, in Santa Monica, California. AP Photo -
Traffic cones line around the empty parking lot of Dodger Stadium, a mass COVID-19 vaccination in Los Angeles. California has closed some vaccination centers and delayed appointments following winter storms elsewhere in the country that hampered the shipment of doses. AP Photo -
Grace John, who works at a school in San Lorenzo, gets a COVID-19 shot at a mobile vaccination clinic run by the Federal Emergency Management Agency and the state in Hayward, California. AP Photo -
U.S. President Joe Biden wears a protective mask while speaking as U.S. Vice President Kamala Harris, left, listens during a meeting with labor leaders on coronavirus relief in the Oval Office of the White House in Washington, D.C., U.S. Biden said last night in a CNN town hall event that Moderna Inc. and Pfizer Inc. agreed to sell more doses of their coronavirus vaccine to the U.S. faster than planned after he invoked federal law that could force their production. Bloomberg -
Boxes containing the Moderna COVID-19 vaccine are prepared to be shipped at the McKesson distribution center in Olive Branch, Mississippi, U.S. Reuters -
After getting the Moderna vaccine, employees wait to be sure they don't have any reactions to the drug before leaving, as Los Angeles Unified School District's first vaccination site opened at the Roybal Learning Center, in Los Angeles. AP Photo -
The Walmart Mobile app is seen on a mobile phone at a Walmart pharmacy where the Moderna coronavirus disease vaccine is available as Walmart and other major U.S. pharmacies take part in the Federal Retail Pharmacy Program, to increase vaccinations in the U.S. in West Haven, Connecticut, U.S. Reuters
Another approach is to give a booster shot of an amended vaccine aimed specifically at these new variants.
Clinical trials are under way to find the best approach to using boosters.
Who is testing a third shot and why?
Pfizer and BioNTech said on February 25 that they were testing a third dose of their jointly developed vaccine in the hope that, by strengthening the immune response against the virus, recipients would be better protected against new variants.
People who had two doses during previous early US clinical trials are being offered a third shot of the companies' messenger RNA (mRNA) vaccine six to 12 months after receiving the initial doses.
This third dose is the same as the first two.
Researchers will later analyse whether extracts from participants' blood, which should contain increased quantities of antibodies against the virus, can "neutralise Sars-CoV-2 strains of interest".
Moderna also revealed last week that it was testing the effects of a third dose of its existing mRNA Covid-19 vaccine against “variants of concern”.
This booster shot is half the strength of the first two doses.
Are any booster shots different to the original Covid-19 vaccine?
Several Covid-19 vaccine developers are reformulating existing shots to cope with new variants. Amended vaccines might be administered as boosters to previously immunised people.
"Efforts are under way to develop a new generation of vaccines that will allow protection to be redirected to emerging variants as booster jabs, if it turns out that it is necessary to do so," said Prof Sarah Gilbert, the University of Oxford scientist who leads researchers working on the Oxford-AstraZeneca vaccine.
Similarly, Moderna is trying out a booster designed to cope with the South African variant, to be used at half or less the strength of the original two doses.
The company is also testing a shot containing its original vaccine and the amended version together, again as a booster at half or less the original strength.
In addition, the amended vaccine and the vaccine mix are each being trialled as standalone vaccines.
Pfizer and BioNTech developed "a potential new variant-specific vaccine" that is likely to be used as a booster.
The companies are talking to drug regulators about trials that would fast-track approval for the shot in a similar way to the system for new influenza vaccines.
Recently announced trials involve testing boosters after initial immunisation with a vaccine from the same company.
However, other trials are happening in which a single dose of the Oxford-AstraZeneca vaccine is being tested with a single dose of either Russia's Sputnik V or the Pfizer-BioNTech vaccine, raising the likelihood that three-dose regimes involving vaccines from different developers might be tested in future.
Are there health risks in having several doses?
Safety is one factor being analysed in the Pfizer-BioNTech and Moderna trials of booster shots.
It is thought unlikely that having three doses of a coronavirus vaccine would create health problems.
“I don’t think you can see any harm,” said Prof John Oxford, emeritus professor of virology at the University of London.
“You might worry that you would get too much of an [immune system] response, but I don’t see any reason why that should be possible.”
He said some childhood immunisations were given routinely in several doses without causing safety issues.
For example, children in the UK receive five doses against tetanus, diphtheria and polio, administered when they are aged eight weeks, 12 weeks, 16 weeks, three years four months, and 14 years. Several other childhood vaccines are also given in several shots, some at the same time as the tetanus, diphtheria and polio vaccines.
