COVID Vaccination Impacts on Transmissibility

A new study has emerged comparing infectious viral load (IVL) across unvaccinated and vaccinated patients with different strains of COVID. The findings show that for some variants, vaccination results in a significantly reduced viral load. However, for Omicron, only boosted individuals show the same result.

Viral load is an important indicator of transmissibility, so these outcomes have important implications for public health policy and individual vaccination choices.

The Study

Published in early April, the study was carried out by Swedish scientists, led by Dr Olha Puhach. COVID transmission is determined partly by infectious viral load, expelled as droplets and aerosols – i.e. coughing and mucous.

So, a higher viral load can serve as a proxy for increased transmission. Still, the transmission process is complex, and many factors can influence it, including behaviour, comorbidities, age, the COVID variant, and which vaccine (if any) is used.

Puhach et al. looked at RNA levels and infectious virus levels to analyse viral loads in their subjects. IVL was then compared across unvaccinated, fully-vaccinated (two doses), and boosted (3 doses) patients, infected with pre-variant of concern (VOC), Delta, and Omicron COVID strains.

565 subjects were split across these three variants, and then further by vaccination status for Delta and Omicron. All pre-VOC patients were unvaccinated, as vaccines were not yet widely available at the time of their infection. All subjects were mildly symptomatic, and of similar age and sex. Samples were taken over a 5-day period post-onset of symptoms. Almost all patients were immunocompetent.

The Results

For patients infected with Delta COVID, vaccination reduced noticeably infectious viral load. Comparing RNA genome copies – the first indicator used – showed unvaccinated Delta-infected subjects had much higher levels. The difference in infectious virus levels – the second indicator – was even more pronounced. At 5 days after the onset of symptoms, infectious virus was detectable in 53.8% vaccinated, and 84.6% unvaccinated patients.

Various studies have emerged over the past few months demonstrating the importance of boosters for the Omicron variant.

But the story was a little different for Omicron. Breakthrough Omicron infections in fully-vaccinated patients had similar levels of both RNA and infectious viral levels, unlike Delta. However, Omicron-infected subjects that had received boosters showed significantly lower infectious viral levels than unvaccinated subjects. This correlates with previous studies that showed two doses of a vaccine had limited efficacy against symptomatic Omicron.

For both Delta and Omicron, there was no significant correlation between the time that had passed since vaccination and IVL.

The Implications

The primary takeaway is that vaccination has benefits for transmission, not just severity of disease. And for Omicron – now the dominant COVID strain worldwide – boosters are crucial to reaping these benefits. As the authors put it, “Our findings highlight the beneficial effect of vaccinations beyond the individual protection from severe disease and underscore the importance of booster vaccination.”

Another important finding of the study was that Omicron-infected subjects showed a lower IVL than Delta-infected. This is surprising, considering Omicron is more infectious than Delta. The implication is then that Omicron’s hyper transmissibility is down to factors other than infectious viral load.

Cover photo by Mufid Majnun on Unsplash.

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