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After nearly nine months of coronavirus restrictions, there may be a light at the end of the tunnel.
The UK is the first country in the world to approve Pfizer’s jab for widespread use, with the NHS “standing ready” to begin immunising care home residents and staff next week.
With an effective vaccine long been hailed a route back to life as we knew it, many are undoubtedly wondering how many people have to receive the jab to achieve herd immunity, freeing us of future lockdowns.
Watch: What is herd immunity?
Herd immunity occurs when an infection cannot take hold in a community. It can come about in two ways – if a sufficient number of people have overcome a virus and are now immune, or via vaccination.
In March, the UK’s chief scientific adviser Sir Patrick Vallance told ITV News: “When you look at infections across whole communities, when you get up to about 60% who’ve had it, you get something like herd immunity, which means we’re then all a bit protected from it.
“So if it does get to that level, that provides quite a lot of protection going forward”.
This was before officials rethought their strategy to just suppress the outbreak to create herd immunity, with a nationwide lockdown being introduced 10 days later.
Also speaking in March, Professor Martin Hibberd, from the London School of Hygiene & Tropical Medicine, said: “When about 70% of the population have been infected and recovered, the chances of outbreaks of the disease become much less.
“In a good scenario, the 70% infected, recovered and immune would be people who were expected to have mild disease and the 30% who were vulnerable to severe disease would be protected by this herd immunity.”
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Eight months later, the bar for herd immunity was thought to be higher.
In November, Professor Julian Savulescu, from the University of Oxford, said: “The exact percentage of the population that would need to be immune for herd immunity to be reached depends on various factors, but current estimates range up to 82%.”
The rate of immunisation required to achieve herd immunity varies between viruses, depending on how infectious they are and how effective a vaccine is.
With measles for example, 95% of people need to be immunised for everyone to be protected.
When asked about the number of Pfizer vaccines needed to achieve herd immunity, the firm’s UK country manager Ben Osborn said: “It’s not for Pfizer to say how many should be vaccinated.
“That’s why we have the independent group JCVI [joint committee on vaccination and immunisation].”
Professor Ralf Rene Reinert – vice president of Pfizer Vaccines – agreed, adding herd immunity cannot be “speculated” at this stage, but could be gauged by looking at the rate of protection in cities that have endured large outbreaks, like those in northern Italy.
“Hopefully I can answer this question at the end of next year,” he added.
Not everyone can be vaccinated, for example people with a suppressed immune system as a result of chemotherapy. They therefore rely on others to get immunised to create herd immunity.
The NHS is said to be preparing for a vaccine rollout like never before. With more than 66 million living in the UK, however, it may be some time before Prof Savulescu’s estimated 82% herd immunity rate can be achieved.
No single pharmaceutical company can produce a sufficient number of vaccine doses to immunise the global population against the coronavirus.
As well as Pfizer’s jab – which it developed with the German biotechnology firm BioNTech – the pharmaceutical giant Moderna and a University of Oxford-AstraZeneca collaboration also have vaccine candidates in the late stage of development, with the government pre-ordering millions of doses of both.
Read more: Concerns over Oxford’s coronavirus vaccine
Prior to Pfizer’s approval, the UK pre-ordered 40 million doses of the jab, enough to vaccinate 20 million people as part of the two-dose regimen.
The government has promised 800,000 doses of the Pfizer vaccine will be available next week, while the “bulk” of the jabs will be administered in 2021.
The first stage of the Pfizer vaccine programme has been announced according to the nine most vulnerable groups of people, starting with care home residents and staff, followed by frontline workers and those over 80, all the way down to people in their fifties.
Aside from the number of people who are required to have the vaccine, herd immunity also depends on what the jab achieves and how long immunisation lasts.
The Oxford-AstraZeneca study is unique in that participants undergo nasal and throat samples once a week, allowing scientists to gauge the number of people who catch the coronavirus but develop no symptoms.
Following vaccination, the scientists noted a lower rate of asymptomatic cases. These individuals may be a key driver of transmission, with a lack of symptoms meaning they do not know to isolate.
“If the vaccine reduces transmission – i.e. vaccinated individuals have fewer asymptomatic infections, or their viral load is lower if they are infected, or if they shed virus for a shorter period of time – the vaccine could make an important contribution to herd immunity,” said Professor Eleanor Riley from the University of Edinburgh.
It is unclear whether the Pfizer and Moderna jabs have the same effect.
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“Pfizer is planning additional studies on how we can document against carriage and transmission of the virus,” said Professor Reinert.
“This is not easy to study. Our focus was protection against disease.”
On 9 November, Pfizer revealed its two-dose vaccine candidate appears to be more than 90% effective at warding off the coronavirus when given 21 days apart.
Long-term data is lacking for all of the vaccines, leaving doctors and officials alike unclear exactly how long the jabs may protect against the coronavirus for.
Speaking of the Oxford-AstraZeneca vaccine’s durability, researcher Professor Andrew Pollard said: “It’s too early to say. We only gave second doses in the UK in August.” The ongoing trial is also being carried out in Brazil and South Africa.
The same lingering question surrounds the Pfizer and Moderna jabs.
Watch: Can you catch coronavirus twice?