The recent news that possibly two COVID-19 vaccines are on the horizon sound encouraging, and might make the prospects for herd immunity closer to reality.

What is herd immunity?

Herd immunity refers to an indirect form of protection against infectious and contagious diseases that is achieved when a significant proportion of the population is immune, either through vaccination or a previous infection. As the WHO’s Chief Scientist, Dr Soumya Swaminathan explained, it kind of works like a protection barrier of immune people that break the transmission chain, making the disease less likely to spread.

When a certain threshold of immunization is reached within a population, even those who are not immune are indirectly protected by the “herd effect”. This threshold depends on how contagious a disease is: for instance, while polio needs about 80% of the population to be immune for the herd effect to work, measles, which is more contagious, requires an immunization of 95%.

Herd immunity refers to an indirect form of protection against infectious and contagious diseases that is achieved when a significant proportion of the population is immune, either through vaccination or a previous infection.

So, what’s all the fuss over herd immunity for COVID-19 about?

According to Dr Swaminathan, herd immunity against COVID-19 would require about 60-70% of the population to be immune. Since a vaccine is not yet available, this would only be achievable by letting the virus spread freely. However, this would come at a great human cost, giving rise to lots of cases of severe illness, which would not only be a burden to healthcare systems, but also substantially increase mortality. As the Director-General of the WHO, Dr Tedros Adhanom Ghebreyesus, recently said in a press briefing, this approach would be “ethically problematic”. Thus, the organization warns that herd immunity should only be discussed in the context of a vaccine.

Game-changing clinical trial results – is the end of the pandemic near?

Very promising news have recently raised hope for the availability of an effective vaccine and for the safe acquisition of herd immunity sooner rather than later. Soon after Pfizer and BioNTech revealed that their vaccine candidate demonstrated more than 90% efficacy in preventing COVID-19, Moderna announced an efficacy of near 95% on their contender. Both candidates rely on a two-dose vaccination schedule and are based on a relatively new technology, in which a molecule called messenger RNA (mRNA) instructs our cells to produce a specific SARS-CoV-2 protein, preparing the immune system for a possible encounter with the actual virus.

Pfizer and Moderna announced that their COVID-19 vaccine candidates were highly effective in phase 3 clinical trials.

Remarkably, early data from both Pfizer’s and Moderna’s final-phase clinical trials revealed that the vaccinated volunteers were less frequently infected with SARS-CoV-2 compared to the placebo group. In particular, Moderna’s vaccine showed a slightly superior efficacy: of the total 95 confirmed cases of COVID-19, only 5 occurred in the vaccinated volunteers. Overall, both companies disclosed that it is possible to achieve protection against this new disease, with an efficacy well above the 50% minimum required by the Food and Drug Administration (FDA). These preliminary results are very encouraging and, in the words of the BioNTech co-founder and CEO, Prof. Ugur Sahin, marked a “milestone (…) on our path towards ending this pandemic”.

So far, no safety concerns have been reported by Pfizer, although further data will continue to be collected in this regard, as well as for additional evidence of efficacy. The Moderna vaccine also seemed to be well tolerated by the participants, with most of the adverse effects being mild or moderate. The companies plan to request an Emergency Use Authorization in the upcoming weeks. By then, the volunteers will have been monitored for at least two months after the last dose of the vaccine was administered, as recommended by the FDA. Both competitors expect to have several million doses available by the end of the year, and up to 1 billion doses in 2021.

Still, despite being very favorable, these results are not conclusive, as long-term data remains to be collected, and will need to be reviewed by the scientific community. Moreover, some important questions remain unanswered. In particular, it is not known how long the protection provided by these vaccines lasts. Cases of reinfection have already been reported. While in some cases symptoms are milder after a second infection, some patients show more severe symptoms. These differences regarding the severity of the symptoms might be related with the dose or with the version of SARS-CoV-2. In fact, the virus might mutate and become more virulent or change its structure in such a way that antibodies produced during the first infection are not be as effective at neutralizing it as before. This could impact the frequency at which we would need to get a vaccine in order to stay protected. For example, we need to get vaccinated against flu every year because the virus that causes it changes rapidly, and the vaccine gets updated yearly to account for these changes.  

As the Phase 3 studies continue, Pfizer and BioNTech will also explore the potential of the vaccine to protect against severe disease or reinfection with SARS-CoV-2. Importantly, Moderna already revealed that there were no cases of severe disease among the immunized individuals, in contrast with 11 cases that developed in the placebo group. In addition, it seems that the vaccine developed by Moderna will be easier to store than the one from Pfizer and BioNTech.

Meanwhile, even though a vaccine seems to be the “only lasting exit strategy from the pandemic”, as the European Commissioner for Health and Food Safety, Stella Kyriakides, has recently said, we will need to continue following measures that have proven to be effective to contain the spread of the virus, such as the use of masks and frequent hand washing, “even when a safe and effective vaccine is (…) available” and until “a sufficient part of the population” is immunized.