Will the spread of the Delta variant prevent herd immunity?

Since the SARS-CoV-2 epidemic has become a pandemic, scientists are hoping that following an effective vaccination campaign, people could develop herd immunity against the virus.

Write in the journal Immunity, Haley E. Randolph, a Ph.D. candidate at the University of Chicago, IL, and Dr. Luis Barreiro, associate professor in Chicago, say 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 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 spread to the UK and other countries around the world, is highly transmissible, potentially reducing the effectiveness of COVID-19 vaccines.

This seems likely to be confirmed after The data which the academics at Imperial College London, UK, responsible for the REACT 1 study published in preprinted form, ahead of peer review.

In the latest round of the study, which randomly tests 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-vaccines. 19 to 49%.

This is what prompted Professor Pollard to suggest that collective immunity is now “not a possibility”. If SARS-CoV-2 can still spread to a significant number of fully vaccinated people, then those who do not have an immune response to the virus are vulnerable to infection.

According to Prof. Pollard, “[t]The problem with this virus is [it is] not measles. If 95% of people have been vaccinated against measles, the virus cannot be transmitted to the population.

“The Delta variant will still infect people who have been vaccinated. And that means anyone who is still not vaccinated at some point will encounter the virus. […], and we have nothing that can [completely] stop this transmission.

– Prof. Andrew Pollard

Talk to MNT, Teacher. Sheena Cruickshank, of the Division of Infection, Immunity & Respiratory Medicine at the University of Manchester, UK, stated that “[h]Erd immunity generally refers to the proportion of a population that must be immunized against an infection to protect those that cannot be immunized, 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, [that is,] you cannot get the infection you were vaccinated against.

“With [COVID-19], we do not get proof of complete sterile immunity for all individuals, either from the vaccine or from a previous infection – rather, for many it seems [to be] partial protection against symptomatic illness or serious illness, ”she added.

“[As] Thus, vaccinated people can still potentially be infected and transmit [the] viruses to others. [However,] there is clear evidence that vaccinated people transmit much less virus than unvaccinated people. “

Prof. Pamela Vallely, also from the Division of Infection, Immunity & Respiratory Medicine in Manchester, said MNT that Professor Pollard is right when he considers that collective immunity is no longer possible.

“The key point is that the vaccine does not stop the transmission of the Delta variant, and other variants – which may also escape full immunity – are likely to arise as long as we have a lot of viral replication across the virus. world, “she said.

Nonetheless, Professor Cruickshank and Professor Vallely say there are still some bright spots, based on the latest data.

Professor Cruickshank said that “[t]The vaccine’s efficacy was slightly reduced to the Delta variant but remains high enough to provide a good level of protection against symptomatic and serious illnesses. [This] chime with [the REACT 1] report [that showed] the majority of hospitalizations concern unvaccinated people.

According to Prof. Vallely, “[w]We can always be optimistic locally because the vaccine stops serious illnesses in most people. So while it still appears to be able to infect and replicate in some vaccinated people, the vast majority of those people do not get sick, or at least do not get very sick. “

The effectiveness of vaccines in protecting against severe COVID-19 is one of the reasons Professors Cruickshank and Vallely believe that booster shots are only necessary for people who are very vulnerable to the disease.

Professor Cruickshank said that “as we have evidence of an immune memory which is long lasting and will protect against symptomatic infections and reduce transmission […], it does not seem necessary at this stage to give boosters to the over 50s, as it is considered, although this needs to be reviewed as more data [come] in.”

Another factor is whether doses of the vaccine should be used as boosters when the vast majority of people in poorer countries have not yet received 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 many populations around the world are fully vaccinated, a position supported by Professor Cruickshank in a recent report. opinion piece.

For Professor Vallely, “it would be beneficial for everyone around the world to reduce the level of replication of the virus on a global scale. 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 further out of vaccine control. “

“I am not qualified to make ethical judgments, but it makes scientific sense to vaccinate as many people as possible around the world to control the pandemic,” Professor Vallely said.

For live updates on the latest developments regarding the novel coronavirus and COVID-19, click on here.