Live updates: follow the latest news on Covid-19 variant Omicron
The Omicron variant is spreading around the globe, disrupting travel and leading to new restrictions being imposed.
Governments are grappling with the surge, with many rolling out Covid-19 vaccine booster drives.
Israel, for instance, is acting quickly to limit its effects by rolling out a fourth coronavirus dose for some citizens.
The country has already administered third doses to more than four in 10 of its population and its experts this week recommended a fourth dose for the over-60s and healthcare staff.
It raises the question of how long it will remain necessary to give boosters. Will they be an annual or twice-annual requirement, or will a few boosters be enough to build lasting immunity?
What can boosters do and where have they been used?
Boosters help the body to strengthen its immunity against coronavirus after it wanes in the months after a previous dose. It can increase the number of neutralising antibodies, the immune cells that attack and disable the virus.
This is seen as particularly important because the Omicron variant contains dozens of mutations and is better able to evade protection from vaccines, meaning stronger immunity is needed to fend it off.
Boosters have already been rolled out, at different rates, in dozens of countries in most regions of the world except sub-Saharan Africa.
Chile is the current leader, with 53 per cent of its population having received a booster, while other nations with significant booster coverage include Iceland (52 per cent), Israel (46 per cent), Uruguay (43 per cent), the UK (43 per cent), Denmark (35 per cent), Malta (34 per cent) and the UAE (33 per cent).
Earlier this month, Albert Bourla, the chief executive of Pfizer, which developed a Covid-19 vaccine with the German company BioNTech, said nations may need to provide fourth doses.
Israel is now forging ahead with this.
Some countries, such as the UK, have offered a fourth shot to people who are considered to be medically more vulnerable to the virus
Will boosters be needed indefinitely?
Immunity against coronaviruses in general does tend to wane over time, said Dr Andrew Freedman, an infectious diseases specialist at Cardiff University in the UK, so boosters are administered to increase antibody levels.
Even if antibody levels fall, a person may have immunity because the immune system has a “memory” created by previous infection or vaccination.
This can be mediated by memory B cells, which are white blood cells that trigger the rapid production of antibodies against a particular antigen (such as SARS-CoV-2, the coronavirus that causes Covid-19).
Also important are memory T cells, another type of white blood cell, which remain in the blood for long periods after vaccination or the clearing of infection, and can mount a rapid response against the same antigen.
Dr Bharat Pankhania, a senior clinical lecturer at the University of Exeter in the UK and consultant in communicable disease control, said that with some infections, such as Hepatitis B, a person may need five or six vaccine doses before they have long-term protection.
“With respect to the coronavirus, we don’t know whether they will need four or five, or a reformulated vaccine,” he said.
“With coronaviruses, we may need quite a few before we [achieve] sustained immunity, if we do.”
At the moment, he said “nobody knows” if continued regular boosters will be needed.
How will virus evolution affect the need for boosters?
The virus will in part determine how long boosters are required for. If it becomes less virulent, the need for additional doses may lessen.
With the Omicron variant, scientists may have detected early signs that the virus is becoming less harmful to infected individuals, although infectiousness has increased.
If the Sars-CoV-2 virus evolves until it causes merely common cold symptoms, boosters are unlikely to be needed, said Dr Freedman. There are currently four coronaviruses that cause colds.
“If it remains a serious virus with serious hospitalisation and death risk, an annual booster will be needed,” he said.
New variants may be better able to evade the protection of existing vaccines, which increases the need for boosters to strengthen immunity.
Such variants have also led vaccine developers to work on reformulated vaccines, which could in future be given as boosters.
While new vaccines may be necessary to cope with new variants, Dr Pankhania said, it was important that parts of the world in which only a minority of people have had a Covid-19 dose received supplies.
“Instead of continental Europe or America giving fourth or fifth doses, why don’t we help Africa?” he said.
Will things be different with the next generation of vaccines and people?
Even if today’s adults need repeated boosters, this may not be the case for the youngest in society.
“What we don’t know is the extent to which people who’ve been exposed from childhood will have a better long-term response than people exposed only in adulthood. But we’re looking decades down the line,” said Prof David Taylor, emeritus professor of pharmaceutical and public health policy at University College London.
Another factor that may change the need for boosters is the capability of next-generation vaccines. Prof Taylor said that while we may need boosters for now, this may not be the case for later vaccines, which may offer more durable immunity.
“Probably we’ll need boosters [for the moment], but maybe the intervention of second and third-generation vaccines will mean that’s no longer necessary,” he said.
The hope also is that later vaccines may also be more effective at stopping transmission of the virus as well as preventing serious illness.