Coronavirus Research Tracking - 15 April
Effectiveness of different numbers of vaccine doses, mutations in the virus or people that affect risks, and low risk of severe Covid in young people
This week, vaccination reduces transmission, and papers demonstrating that four doses can be better than three, which are better than two, which are better than natural immunity.
Viral mutations that enable escape from T cell recognition, identifying human mutations that increase or decrease infection or disease susceptibility, the rarity of severe Covid in young people, and the effectiveness of genome surveillance in NZ to control the Delta outbreak.
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.
Vaccination can lower transmission risk
A Swiss study has found that vaccinations lower transmission risks. Two mRNA vaccine doses significantly reduces the infectious viral load from a Delta variant infection. For Omicron, three doses were required to reduce the infectious viral load.
Lab experiments determined not just the amount of virus in samples, but what proportion was able to infect other cells (the infectious viral load). Infectious viral loads were significantly lower than overall viral load (assessed by number of viral genomes).
Infectious viral loads from Omicron infections were lower than those for Delta, so the authors suggest that other factors must contribute to Omicron’s greater transmission rate. Viral samples were collected from 565 people within the first five days of symptomatic infections. The paper was published in Nature Medicine.
More evidence of short term benefits of four Pfizer doses in older people
Compared with three Pfizer/BioNTech doses, a fourth dose provides much greater protection against hospitalisation, severe Covid, and death over the short term in older people. Outcomes were compared between patients with Covid who had received a third dose at least four months earlier, with similar patients who had had a fourth dose within the previous month. Infection risk was also lower after the fourth dose. All study participants were over 60. The paper was published in the New England Journal of Medicine.
Stronger immune response seen after third vaccine dose than after two
Three months after a third mRNA vaccine dose the immune response showed a larger magnitude, potency and breadth, compared with two doses. A similar response was also seen in people who had two doses and a subsequent (pre-Omicron) infection. This resulted in stronger neutralisation of the Omicron variant in lab tests. The study was relatively small, involving 99 participants. The paper has not yet been peer reviewed.
Natural immunity provides poor protection against Omicron
A pre-Omicron Infection without vaccination doesn’t provide good protection against the Omicron variant. Much higher neutralising antibody levels are seen in people who received two Pfizer doses after an infection. One vaccine dose after infection did not substantially increase neutralising antibody levels. The study involved testing sera from 65 participants. The paper was published in Med.
Some viral mutations may prevent some T cells from recognising it
Some SARS-CoV-2 mutations may allow the virus to escape from CD8+ T cell binding. An analysis of viral mutations and T cell epitopes identified 83 viral mutations that may decrease CD8+ T cell recognition of the virus. Laboratory tests are needed to confirm this. The paper was published in Vaccines.
Identifying human alleles that increase or decrease susceptibility
A News & Views article in Nature Genetics discusses two papers in the same issue that identify allelic variants in people that are associated with greater susceptibility (or resistance) to SARS-CoV-2 infection, or development of more severe Covid-19.
The article emphasises that well designed case-control studies like these two papers are necessary to demonstrate strong associations between phenotypes and viral susceptibility or resistance. It concludes by noting that insights provided by genome-wide association studies can help develop new forms of treatments that may help prevent hospitalisation, or improve the care of those with more severe Covid-19.
Severe Covid very rare in young people
A study in England found that very few children developed severe Covid-19. Over 14 months only 49 hospitalisations of under 18 year olds at one London hospital were judged to be at least partly due to Covid-19 symptoms, and only 15 had severe Covid-19.
More hospitalisations, and severe Covid-19 cases, were associated with the Alpha variants than either the Delta or Omicron infections, despite the latter two infection waves causing higher infection rates. The paper was published in the Journal of Infection.
Using genomics to support transmission control in NZ
A New Zealand study shows how genomics was used to inform control measures during the Delta outbreak. Mutations in one open reading frame in the virus allowed undetected transmission events to be identified and linked, aiding control of the outbreak. The paper has not yet been peer reviewed.