Written by Timothy Huzar
Professor Andrew Pollard, director of the Oxford Vaccine Group, says herd immunity is "not a possibility" in light of the spread of the Delta variant of SARS-CoV-2.
Professor Pollard, who is also the chairman of the Joint Committee on Vaccination and Immunisation, which advises the UK government, was testifying to members of the UK House of Commons.
His comments follow the preprint release of the latest data from the Real-time Assessment of Community Transmission 1 (REACT 1) study, which suggests that COVID-19 vaccines are only 49% effective against the Delta variant of SARS-CoV-2.
Herd immunity
Since the SARS-CoV-2 outbreak became a pandemic, scientists have hoped that, following an effective vaccination campaign, populations will be able to develop herd immunity to the virus.
Writing in the journal Immunity , Haley E. Randolph, Ph.D. candidate at the University of Chicago, IL, and Dr. Luis Barreiro, associate professor at Chicago, say that herd immunity describes a situation in which so many individuals in a population are immune to a virus that it stops spreading and may even begin to decline.
As a result, even people who do not have an antibody response to the virus have some degree of protection.
However, scientists are concerned that the Delta variant of SARS-CoV-2, which has been spreading in the UK and other countries around the world, is highly transmissible, which could reduce the effectiveness of COVID-19 vaccines.
The Delta variant problem
This is likely to be confirmed by the data that academics from Imperial College London, UK, in charge of the REACT 1 study, published in a pre-printed form, before peer review.
In the latest round of the study, which randomly tested up to 150,000 people in England for COVID-19, scientists found that the Delta variant was completely dominant and had reduced the effectiveness of COVID-19 vaccines to 49%.
This is what led Professor Pollard to suggest that herd immunity is now "not a possibility." If SARS-CoV-2 can still spread to a significant number of fully vaccinated people, those without an immune response to the virus are vulnerable to infection.
According to Professor Pollard, “the problem with this virus is not measles. If 95% of people were vaccinated against measles, the virus could not be transmitted in the population.”
Speaking to Medical News Today , Professor Sheena Cruickshank, from the Division of Infection, Immunity and Respiratory Medicine at the University of Manchester, UK, said that “hereditary immunity generally refers to the proportion of a population that needs to be immune to an infection in order to protect those who cannot be immune, and vaccination is the way to achieve this, as it is the safest.”
"For true herd immunity, you need a good level of vaccination and evidence of sterile immunity, meaning you cannot contract the infection you have been vaccinated against."
“With [COVID-19], we are not getting evidence of complete sterile immunity for all individuals, either from the vaccine or from a previous infection; rather, for many, there appears to be partial protection against either symptomatic or severe disease.”
“[As] such, vaccinated people can still potentially become infected and transmit [the] virus to others. [However,] there is clear evidence that vaccinated people transmit far less virus than unvaccinated people.”
Professor Pamela Vallely, also from Manchester's Division of Infection, Immunity and Respiratory Medicine, told MNT that Professor Pollard is correct in his assessment that herd immunity is no longer possible.
"The key point is that the vaccine does not stop transmission of the Delta variant, and other variants are likely to emerge, which will also be able to evade total immunity, as long as we have a lot of viral replication around the world."
Prevent serious infections
However, both Professor Cruickshank and Professor Vallely state that there are still positive aspects, according to the most recent data.
Professor Cruickshank said that “the vaccine’s effectiveness has decreased slightly in the Delta variant, but it is still high enough to provide a good level of protection against symptomatic and severe illness. This is consistent with the REACT 1 report, which showed that the majority of hospitalizations were in unvaccinated individuals.”
According to Professor Vallely, “we can still be optimistic at a local level, as the vaccine is preventing severe illness in most people. So, although it appears that it can still infect and replicate in some vaccinated individuals, the vast majority of those people do not become ill, or at least do not become seriously ill.”
Booster shots?
The effectiveness of vaccines in protecting against severe COVID-19 is one of the reasons why Professor Cruickshank and Professor Vallely believe that booster vaccines are only necessary for people who are highly vulnerable to the disease.
Professor Cruickshank said that “as we have evidence of immunological memory that is long-lasting and will protect against symptomatic infection and reduce transmission […], it appears there is no need at this stage to give boosters to those over 50, as is being considered, although this should be reviewed as more data comes in.”
Another factor is whether the vaccine doses should be used as boosters when the vast majority of people in the poorest countries have yet to receive their first dose.
Professor Vallely pointed out that, in addition to any ethical considerations regarding vaccine distribution, there is a medical basis for ensuring that as many populations as possible worldwide are fully vaccinated, a position supported by Professor Cruickshank in a recent opinion piece.
Professor Vallely commented: “Reducing the level of virus replication globally would benefit everyone worldwide. As long as there are high levels of transmission, there is a high level of replication and, therefore, more opportunities for the virus to mutate into a more severe form and/or one further beyond the control of vaccines.”
"I am not qualified to make ethical judgments, but it makes scientific sense to vaccinate as many people as quickly as possible worldwide to control the pandemic."