COVID-19 and vaccines: Can the transmission of the virus be stopped?

by Antonino Napoleone, Giuseppe Scarlata | December 02, 2021.
edited by Ivy Rose Sebastian.

Whether or not one believes in vaccines, is a matter of opinion, but data and experiments incontrovertibly show that vaccinations reduce the risk of COVID-19 transmission. However, researchers are still struggling to find an unequivocal answer concerning two factors: The first is the statistical factor, which would quantify the actual risk of transmission of COVID-19 between vaccinated and unvaccinated individuals and the mix of the two categories. This is not particularly easy to decipher, as there are many variables involved, such as the variants of the virus, the immune system, the type of vaccine, and the number of booster vaccinations, to mention just a few. The second factor concerns time, or in other words, the duration of the vaccine’s protective capacity. These factors are complex and are what most epidemiological studies around the world are focusing on now. While we eagerly await at the end of this convoluted tunnel, let us analyse some data that is already available, and try to understand how vaccination can reduce the viral load and decrease the risk of transmitting SARS-CoV-2, but above all what effect the spread of new virus variants will have over the coming months.  

Relationship between infection and transmission

To begin with, it is crucial to understand the relationship between infection and viral transmission. The link between the two factors applies to all infectious diseases and consists of identifying the type of relationship that is established between the virus and the host. Every day we encounter millions of viruses, but this does not necessarily imply the development of a distinct symptomatic disease. In most cases, our immune system can easily ‘cleanse’ our body of intruders before they get a chance to take over and cause disease, and this simultaneously reduces or eliminates the risk of transmitting the pathogen to other individuals. When this does not happen, pathogenic viruses, such as SARS-CoV-2, enter the host organism, replicate, alter the state of health with mild or more severe symptoms (COVID-19 syndrome), and spread to the external environment, with various rates of transmission. The viral replicative capacity is a crucial factor for the virus to overcome the immune defences, cause a severe pathological state in the host, and be transmitted to other individuals. Viruses or viral variants that have developed efficient replication systems have a greater evolutionary advantage over other viruses/variants in being more infectious and pathogenic in natural hosts. SARS-CoV-2 has puzzled and alarmed both the lay public and the scientific community precisely because of its rapidly evolving replicative capacity.

Vaccine’s target

The development of vaccines against SARS-CoV-2 has been a tremendous biotechnological success story, on which several detailed articles are available (Read more: The race to the winning vaccine: comparison of adopted technologies, and What is the secret ingredient in Pfizer and Moderna vaccines?). In theory, a vaccine is defined as highly effective when it can prevent infection in vaccinated and immunised individuals, and in addition, it also reduces the risk of transmission of the pathogen. Although several clinical studies have shown that the currently available vaccines can prevent COVID-19, more recent studies have shown that they can also reduce the transmission of the infection. A recent study looked at the effect of vaccination on a cohort of SARS-CoV-2 positive individuals who had received two doses of Pfizer/BioNTech’s BNT162b2 mRNA vaccine compared to a control group of unvaccinated patients. This study found that the viral load was four times lower in subjects who were infected 12-28 days after receiving the first dose of vaccine, compared to the unvaccinated group. A lower viral load means lower infectivity and a lower risk of developing more severe symptoms (Figures 1a, 1b, 1c). Preliminary data released by the pharmaceutical companies guaranteed 95% efficacy in preventing infection from 7 days following the second dose, and partial protection from 12 days after the first dose. The data from the study mentioned above further supports the effectiveness of the vaccine in limiting the spread of infection. 

Vaccination may not be able to completely prevent the chances of infection with SARS-CoV-2, but it has been shown to make infected individuals less contagious, and thus limit the transmission of the virus. Further studies are in progress, and we will soon be able to compare the data.

What about virus variants?

By now, we are all aware that SARS-CoV-2 accumulates mutations and that variants have been circulating for some time. Another study has shown that individuals infected with the Delta variant of the virus are less infectious and become negative earlier if they have already received a full cycle of the Pfizer/BioNTech vaccination when compared to unvaccinated individuals. However, this protective effect has been seen to diminish over time, compared to the Alpha variant from which the vaccines were initially developed and tested. Unfortunately, the Delta variant is spreading rapidly worldwide, and this may explain why there is an increase in its rate of transmission, despite the increasing number of vaccinations. Most recently, a new variant was reported by the WHO, dubbed Omicron, harbouring a large number of the mutations found in other variants, including Delta has also been discovered in South Africa on 24 November. A large number of mutations have been detected in this variant, some of which have caused considerable concern as they could indicate an increased risk of re-infection and escape of the immune response stimulated by currently available vaccines. (Figure 2). Additional data are required to fully comprehend the protective mechanisms of the vaccine, especially regarding the possibility of integrating a third booster dose into the vaccination cycle.

Conclusions

From all the available data, we can conclude that viral load is not only a driving factor in the transmission of SARS-CoV-2, but it is also highly correlated with the course of infection. Symptomatic forms of COVID-19 tend to be associated with a shorter incubation time, higher viral load and higher infectivity. Numerous studies are still in progress to decipher the efficacy and long-term effect of vaccination in this scenario. To date, we know that the vaccine is an effective and safe means of drastically reducing the severity of symptoms, mortality rates and the risk of virus transmission. As we have recently learned with the spread of the new Omicron variant, the dynamics of the pandemic are constantly evolving. But concurrently, where others see scepticism, scientific minds are pushing their limits to bring about innovation and solutions. So, the message to entrust medical sciences, rather than media, must be even stronger as we await new encouraging results!

Moderna Announces Strategy to Address Omicron (B.1.1.529) SARS-CoV-2 Variant:
1. Testing three existing COVID-19 vaccine booster candidates against the Omicron variant.
2. Announcing a new variant-specific vaccine candidate against Omicron (mRNA-1273.529).

Source: Moderna, Inc. Press Releases November 26, 2021.

References:

1. https://www.nature.com/articles/d41586-021-02275-2
2. https://doi.org/10.1038/d41586-021-02689-y
3. Levine-Tiefenbrun, M. et al. Preprint at medRxiv https://doi.org/10.1101/2021.02.06.21251283 (2021).
4. Marks, M. et al. Lancet Infect. Dis. https://doi.org/10.1016/S1473-3099(20)30985-3 (2021).
5. https://www.nature.com/articles/d41586-021-03552-w?utm_source=Nature+Briefing&utm_campaign=d1f63b83a9-briefing-wk-20211126&utm_medium=email&utm_term=0_c9dfd39373-d1f63b83a9-42148423
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