More countries are turning to a once-controversial approach that has now been confirmed by scientific studies: doubling or tripling the time between the first and second Covid vaccine doses.
Delaying the second shot not only helps the current supply of shots to be spread more widely but also increases the defensive strength of the first inoculation by allowing the immune system more time to react to it. According to new research, levels of antibodies developed to combat the virus are 20 percent to 300 percent higher when the follow-up vaccine is given later.
That's good news for countries like Singapore, which is dealing with an unusual, but minor, increase in cases following last year's successful containment of the virus. To meet a target of covering the entire adult population with at least one shot by the end of August, the city-state is now expanding dose periods from three to four weeks to six to eight weeks.
Other countries in similar circumstances, such as those with a scarcity of vaccines and agitated citizens, are likely to follow suit.
"If I could, I would push a button that says right now, this second, we give one dose to everybody we can reach," Gregory Poland, a virologist and director of the Mayo Clinic's Vaccine Research Group. "We'll get around to second doses later."
"In the midst of a world on fire, you put out as many fires as you can, as quickly as you can," Poland said.
There was no reassuring evidence on longer dosing cycles when the vaccine rollout started at the end of 2020. Then, countries limited their use to the most vulnerable people, ensuring that a backup shot was available. During a major outbreak in late 2020, the UK was the first to lift those restrictions, a move that was initially criticized but has since proved to be foresighted.
According to the observations, the first shot primes the immune system, allowing it to begin producing antiviral antibodies. The stronger the response to the second booster shot, which comes weeks or months later, the longer the response is allowed to mature.
Longer dose cycles have been shown to favor all forms of vaccines.
The antibody response was 3.5-fold higher in people over 80 who received a strong mRNA vaccine from Pfizer Inc. and BioNTech SE if the second shot was given after three months rather than three weeks. Other research found that delaying the final shot for nine to 15 weeks reduced hospitalizations, infections, and deaths, while a Canadian study found that a six-month wait provided the greatest benefit.
There are some disadvantages. Because of the longer time between doses, countries may have to wait longer to protect their populations. Although one shot provides some protection, people aren't completely immunized until several weeks after receiving their second dose.
When less potent vaccines are used or more transmissible virus forms are circulating, the period is especially harmful.
Some nations are testing the limits. India's three- to the four-month interval between doses, which is among the world's longest, means that those rushing to get vaccinated during the current outbreak won't be completely safe until the summer or fall.
Though studies indicate a 12-week interval between AstraZeneca shots is ideal, there is little information on the effect of increasing the interval to 16 weeks. In India, the British drugmaker's vaccine now accounts for the vast majority of vaccinations.
The long wait may also make it more difficult to persuade millions of people to return to finish the series, particularly if they believe they already have some coverage from the first clip. People in developed countries are still finding it difficult to return due to transportation and communication issues.
"When you have any kind of a two-dose schedule, it's hard logistically," said Gigi Gronvall, a senior scholar at the Center for Health Security at the Johns Hopkins Bloomberg School of Public Health. "You worry about when people will come back for a second shot, or if you will lose them entirely."
Many places are willing to make this tradeoff, particularly given the scarcity of vaccines. Officials hope to offer the first dose to around 4.7 million people by the end of the summer, thanks to a new strain first found in India and now spreading in Singapore.
"We're vaccinating 40,000 people a day at the moment, so you can do the math and work out how long that'll last," said Dale Fisher, an infectious diseases professor at the National University of Singapore.
"And that's a fairly rigid pipeline. It's not like you can just order more," he added.
Several hundred thousand people would receive their first injection earlier as a result of delaying the second dose, he said.