Coronavirus Research Tracking - 1 October - Vaccine edition

Vaccine effectiveness, correlates of protection, viral loads

This week vaccine-related papers on when vaccine immune does or doesn’t decline, differing antibody responses to different vaccines, correlates of protection for the AstraZeneca/Oxford vaccine, and viral loads in vaccinated people.

Non-vaccine papers are included in a separate post.

The tracker is shared with the COVID-19 Vaccine Media Hub.

The Research Tracker is prepared by Dr Robert Hickson for the Science Media Centre.

Vaccine-related papers

Limited declines over 5 months in vaccine protection against serious Covid-19 for under 65s

Real world data from the UK found limited waning of protection against serious disease and death 20 weeks after vaccination with the Pfizer/BioNTech and AstraZeneca/Oxford vaccines. Declines in protection were greater in those over 65 (compared to 40 to 64 year olds) and those with underlying medical conditions. 

The data are broken down for various post-vaccination time points, age groups and disease severity. 

Effectiveness was greater for Pfizer/BioNTech than AstraZeneca/Oxford. However, the authors note that in the UK the AstraZeneca vaccine was given to a greater proportion of people in aged care and those at greater clinical risk than the Pfizer vaccine, so direct comparisons of effectiveness between the vaccines are not possible. The paper has not yet been peer reviewed

Johnson & Johnson/Janssen vaccine shows strong effectiveness over 5 months

Similar results were found for the Johnson & Johnson/Janssen vaccine. Real world data from the US found it provided 79% protection against infection and 81% against hospitalisation. There was no evidence of a decline in effectiveness between March and July. Effectiveness was higher for those under 50, and for those who were not immunocompromised. The study involved nearly 400,000 vaccinated people. The paper has not yet been peer reviewed

Differences in antibody responses between vaccines

Different vaccines produce different antibody responses against viral variants. This study compared the Pfizer/BioNTech, Moderna, AstraZeneca/Oxford and Johnson & Johnson/Janssen vaccines at four weeks after vaccination. The two mRNA vaccines produced considerably higher neutralising antibody levels against the wild type virus and the four variants of concern than two adenovirus-based vaccines.

Antibody titre levels were considerably lower for all the vaccines for the variants of concern, compared with the wild type virus. Neutralisation titres correlated with reported levels of protection, although the authors note that other immune components also have roles in the level of protection.

The age profiles of vaccinated people providing sera were not the same in the study. Those who had the AstraZeneca vaccine were older, so that may have influenced some results. Neutralisation was only assessed once shortly after vaccination so the results may change over longer time periods. The paper has not yet been peer reviewed

Correlates of protection for AstraZeneca/Oxford vaccine

High levels of IgG antibodies that target the spike protein or receptor binding domain, and neutralising antibodies correlate with stronger protection from symptomatic infection for the AstraZeneca/Oxford vaccine. The results apply only to mild Covid-19. Effectiveness was based on antibody levels measured 28 days after the second dose.

The research identifies titres of these antibodies that provide effectiveness at various levels of protection, although the confidence intervals are often broad. The results are based on 171 infections and over 1,000 uninfected cases, with infections involving predominantly the Alpha variant. The paper was published in Nature Medicine.

Viral loads can be similar in vaccinated and unvaccinated people

Several studies report high viral levels in vaccinated but infected people.

A US study reports that viral loads did not differ between vaccinated and unvaccinated cases, nor between symptomatic and asymptomatic Delta variant infections. This was based on the median number of PCR cycles to yield a positive result and included 869 positive samples. 

There was considerable variation in the cycle threshold number between individuals regardless of vaccination and symptom status. The authors recommend that masks, social distancing and other measures should not be removed or reduced for vaccinated and asymptomatic people during outbreaks.

In the study “asymptomatic” was defined as not having symptoms at the time of testing, rather than never having symptoms. No information was provided on what vaccines were involved. The paper has not yet been peer reviewed.

A Vietnamese study of infected vaccinated healthcare workers found they carried high viral loads, and probably transmitted the virus between each other. They also had lower levels of neutralising antibodies, compared with vaccinated uninfected colleagues. However, antibody levels did not correlate with viral loads or severity of infection.

Sequenced genomes all belonged to the Delta variant. Median viral loads were 251 times higher in symptomatic cases than levels for non-Delta variant infections in unvaccinated people in April and May 2020. Some of the infected workers tested positive for viral RNA for as long as 33 days, suggesting prolonged infections.

Participants were vaccinated with the AstraZeneca/Oxford vaccine, and 62 vaccinated infected workers were in the study. Mask wearing was not mandatory for these healthcare workers at the time. The paper has not yet been peer reviewed

An Italian study also found that infected vaccinated people can have high viral loads. Participants had received the Pfizer/BioNTech vaccine. Infectious virus was found in nasopharyngeal samples from both asymptomatic and symptomatic cases. 

Only 0.3% of 735,000 vaccinated tested positive after partial or complete vaccination. The study was undertaken before the Delta variant was common. The paper has not yet been peer reviewed

No safety surprises after third Pfizer vaccine shot

A small trial (with 300 participants) reports that reactions to a third Pfizer/BioNTech dose were similar to those seen after a second dose. The paper was published in Morbidity and Mortality Weekly Report.

The length of immunity from vaccines

A News Explainer in Nature discusses recent evidence about how long immunity to Covid-19 vaccines lasts, and the advantages and disadvantages of booster shots.

Other SMC resources