Evidence shows the government’s vaccine-obsessed strategy is misguided. Eliminating the Delta variant cannot be achieved with vaccines only but together with mitigation.

First published in August 2021.

Concerning evidence from different sources, including Public Health England (PHE) and Imperial College London’s REACT-1 study showing that while there is around 50% protection against infection from the Delta variant with vaccination, those who do get infected have virus levels that are similar to the unvaccinated and can likely transmit as efficiently.

In the PHE technical report released last week, virus levels (measured by Ct values) were very similar for Delta infections in the vaccinated and the unvaccinated, and were very high (Ct~18) compared to levels with Alpha infections in the past.

Vaccine protection against the Alpha variant was better.

Public Health England

REACT-1 data out a few days ago also echoes this. Protection of vaccination (aggregate values for Pfizer and AstraZeneca) against infection from Delta (49%) was not very different from the protection against infection with high virus levels (Ct<27) (58% protection).

REACT-1, Imperial College London

This means that the risk of infection reduces by about half with vaccination (aggregate for Pfizer and AstraZeneca), but those who get infected have high virus loads, and are likely to be able to transmit as efficiently as those who are unvaccinated.

This also echoes the findings highlighted by the CDC in recent reports from the United States showing similar virus levels (Ct values) among infections in vaccinated and unvaccinated people in outbreaks in Massachusetts.


This was among the reports that led the CDC to change their policy on recommendations of masking for those who were vaccinated. Many countries that had previously eased restrictions for those vaccinated have had to go back on this. This includes Israel, where mask mandates were re-introduced.

Many countries in South East Asia that eased quarantines for those who were vaccinated (South Korea, Vietnam, Taiwan) also had to remove these exemptions after the introduction and rapid spread of the Delta variant.

Delta has changed the course of the pandemic – there is no doubt.

While we are really lucky that vaccines still protect well against severe disease with Delta, the reduced vaccine effectiveness in preventing infection is problematic from a population immunity perspective.

It is important that this evidence is considered in policy. The UK seems to be making the same mistakes – granting exemptions from quarantine, isolation of contacts of cases and allowing activities like nightclubbing based on vaccine status. Given what we are seeing, this will very likely backfire and lead to surges in transmission. Many countries that made exemptions based on vaccination have had to U-turn. Do we have to make the same mistakes, and suffer the consequences?

It also means that we are unlikely to reach herd immunity thresholds even if we vaccinate almost everyone given the effectiveness of our current vaccines against Delta. Vaccinating children will help, given these groups have high levels of exposure and contact. But this may not be enough with Delta to stop outbreaks or even bring R below 1 without continuing mitigations. Governments need to seriously invest in long term options like ventilation in indoor environments – schools, workplaces, shops, businesses.

This is vital to plan for.

These don’t require restrictions, but they do need investment and resourcing from governments. And we know improving ventilation works. Vaccines will not get us out of this on their own – we need to protect them against new variants and add to their protection with mitigation. It means thinking about different infrastructures and setups for buildings in the longer term. The impacts of these will be positive not just for COVID-19 control, but also generally on our future health. 


Dr Deepti Gurdasani, Senior Lecturer in Epidemiology, Statistical Genetics, Machine Learning, Queen Mary University of London.


Going Further:

[This piece was first published as a Twitter thread and turned into the above article on 12 August 2021 with the purpose of reaching a larger audience. It has been minorly edited and corrected, and published with the author’s consent. | The author of the tweets writes in a personal capacity.]

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