What is the real-world impact of COVID-19 vaccines on community transmission?

With more than 60 million doses administered across the UK, including more than 22m second doses, data on the effectiveness of COVID-19 vaccines on virus transmission is increasing. This rapid response will describe the latest data on the effectiveness of COVID-19 vaccines on community transmission and the evidence considered at SAGE with regards to vaccination rollout, lifting of restrictions and impact on infection rate.

What is the latest evidence on COVID-19 vaccines and community transmission?

Clinical trials and real world data have proven that COVID-19 vaccines are effective in preventing symptomatic COVID-19 disease, hospitalisations and deaths (see previous POST’s rapid response on The performance of COVID-19 vaccines in clinical trials and in real world conditions). Since the beginning of immunisation programmes worldwide, data on the impact on transmission has been emerging (see previous POST’s rapid response on COVID-19 vaccines and virus transmission).

An initial study (not peer reviewed) from Public Health England (PHE) based on over 550,000 households in England monitored between January and March 2021 found that people who got infected 3 weeks after receiving one dose of the Pfizer/BioNTech or the University of Oxford/ AstraZeneca vaccine were between 38% and 49% less likely to transmit the virus to their household. According to the latest PHE COVID-19 vaccine surveillance report published on 20 May 2021, effectiveness for one dose of the Pfizer-BioNTech against infection is estimated to be between 55% and 70%, while for one dose of the Oxford-AstraZeneca vaccine it ranges from around 60 to 70%.

This is reflected at the community level. The latest data from the Real-time Assessment of Community Transmission (REACT) study showed that infection rates have halved since March and that around 1 in 1,000 people are now estimated to be infected in England. The study performed PCR tests on 127,000 volunteers between 15 April and 3 May and found that infection rates in people aged 55 to 64-years decreased from 0.17% to 0.06% between March and May, likely indicating an effect of the vaccination programme in this age group. The highest infection rate (0.21%) was among those aged 25 to 34 years.

COVID-19 vaccines and community transmission: international examples

Evidence from other immunisation programmes worldwide support the effectiveness of COVID-19 vaccines in reducing community transmission. However, they also show that different vaccines have different protection levels and that NPIs (non-pharmaceutical interventions) still have a key role to play in controlling disease spread. Israel and Chile can be used as examples to illustrate this (see Figure 1 and Figure 2).

First case study: Israel

Israel started its vaccination programme on 19 December 2020. As of 20 May 2021, almost 60% of its population has received two vaccine doses (see Figure 1). Israel largely used the Pfizer / BioNTech vaccine, an RNA-based vaccine requiring 21 days between the first and the second dose. Israel’s latest exit strategy included three different phases:

1. 7 February 2021: measures included reopening of open-air nature reserves and parks. 25% of the population was fully vaccinated at that point (40% received at least one dose);
2. 21 February 2021: ‘green passes’ (a form of COVID-19 certification status) were implemented to access venues such as gyms and concerts. 35% of the population was fully vaccinated at that point (50 % received at least one dose);
3. 7 March 2021: more restrictions were lifted, including on gatherings of up to 20 people indoors and 50 outdoors. 44% of the population was fully vaccinated at that point (57% received at least one dose).

Many considered Israel’s mass-vaccination campaign a success story that could be a model strategy for other countries. Its slow exit strategy (which still uses some NPIs, such as the use of masks in indoor public spaces) combined with the high effectiveness of the Pfizer/BioNTech vaccine in preventing infections after a single dose, ensured a significant and constant reduction of daily new cases (see Figure 2). There are currently around 3 new cases per day per 1,000,000.

Israel national surveillance data were published on The Lancet on 5 May 2021. The effectiveness of two doses of the Pfizer/ BioNTech vaccine against SARS-CoV-2 infection was estimated to be over 90% (95·3% for symptomatic infection and 91·5% for asymptomatic infection). A recent pre-print (not peer reviewed) analysing vaccination and infection data in Israel also suggests that high vaccine uptake in adults is highly correlated with indirect protection of unimmunised children under 16.

Second case study: Chile

Chile started its vaccination programme on 24 December 2020. As of 20 May 2021, almost 50% of its population has received two vaccine doses (see Figure 1). Chile employed two different vaccines. 93% of the population received the CoronaVac vaccine (an inactivated vaccine produced by the Chinese company Sinovac) and the remaining 7 % received the Pfizer/BioNTech vaccine.

Despite the fast vaccination rollout, Chile faced another wave of COVID-19 cases and on 1 April 2021 announced tighter restrictions, including border closures. There were 363 daily new cases per 1,000,000 (see Figure 2). Only 20% of the population was fully vaccinated at that point (see Figure 1) and over 35% received at least one dose.

This spike in cases in Chile appears to have been a result of:

• the early lifting of restrictions on 30 December to support the Chilean holiday season. These included the reopening of restaurants, shops, and holiday resorts and international travel.
• the lower effectiveness of the CoronaVac vaccine in comparison to the Pfizer/BioNTech vaccine. Data from the Chilean Ministry of Health presented on 29 April 2021 at the World Health Organisation’s extraordinary meeting of the Strategic Advisory Group of Experts on Immunization estimated that the CoronaVac vaccine was only 15% effective against infection after the first dose, rising to 63% effective two weeks after the second dose. Some experts argued that this risk was not made clear to those receiving the vaccine, many of who went on to catch the virus after their first dose.

What has been discussed by the Scientific Advisory Group for Emergencies (SAGE) about the vaccination rollout and the lifting of restrictions?

The success of the COVID-19 vaccination campaign is the first criterion considered in the Government’s COVID-19 Response – Spring 2021 (the Roadmap) to allow the gradual lifting of restrictions. The Scientific Advisory Group for Emergencies (SAGE) has discussed in several meetings the relationship between vaccination rollout, the lifting of NPIs and an increase in transmission:

• During its 79^th meeting on 4 February 2021, SAGE highlighted how, even with a high proportion of the population being vaccinated, lifting NPIs too quickly would lead to an increase in cases. SAGE discussed the advantages of lifting NPIs later, such as allowing continued reduction of cases and more people to be vaccinated. Modelling data commissioned by Scientific Pandemic Influenza Group on Modelling (SPI-M) supported these conclusions.
• During its 80^th meeting on 11 February 2021, SAGE considered a brief on behavioural and social considerations when reducing restrictions produced by the Independent Scientific Pandemic Insights Group on Behaviours (SPI-B). SAGE concluded that it is essential to provide clear messages to people, especially highlighting why it is important to still adhere to certain protective behaviours, as their perception of risk would change as the vaccination campaign progresses.
• During its 83^rd meeting on 11 March 2021, SAGE considered a SPI-B (Scientific Pandemic Insights Group on Behaviours) brief on ‘Behavioural considerations for vaccine uptake in Phase 2 and beyond’, that analysed adherence to NPIs following vaccination. SPI-B concluded that adherence to NPIs did not significantly change following vaccination, although some groups (such as people aged 16-29 years) were less likely to comply.
• During its 86^th meeting on 8 April 2021, SAGE discussed a paper produced by SPI-M (Scientific Pandemic Influenza Group on Modelling) contributors on ‘Ready reckoners under vaccination’. These describe the relationship between different parameters, such as the reproduction number R, the number of contacts made outside home and school, vaccination rollout, and school opening. The analysis highlighted that a greater impact on transmission is expected following vaccination of younger age groups, given that on average they have a higher number of contacts than the older population. SAGE concluded that ‘these ready reckoners will be useful when considering the impact of relaxation of social distancing’.
• During its 88^th meeting on 5 May 2021, SAGE discussed a paper produced by the Scientific Pandemic Influenza Group on Modelling, Operational sub-group (SPI-M-O) entitled ‘Summary of further modelling of easing restrictions – Roadmap Step 3’. SAGE concluded that it is “highly likely that there will be a further resurgence in hospitalisations and deaths at some point”, but that this resurgence will be smaller if measures to reduce contact between people continue and if the vaccine is rolled out to younger adults. Moreover it highlighted that, without any interventions, a new variant that is either more transmissible or able to escape the vaccine-triggered immune response could lead to a larger wave than that observed in January 2021. This could have implications with regards to the B.1.617.2 variant (first identified in India) (see POST Rapid Response COVID-19 vaccines: effectiveness against the B.1.617.2 variant and latest updates from trials).

Acknowledgements

POST would like to thank Professor Adam Finn (University of Bristol; Bristol Royal Hospital for Children) who acted as external peer reviewer in preparation of this article.

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