Delaying second COVID-19 vaccine doses will make supplies last longer but comes with risks

During Pfizer-BioNTech’s vaccine trial, one vaccinated participant and nine who received a placebo developed a severe case of COVID-19 after the first dose. This suggests that participants developed partial protection as early as 12 days after the first dose. However, all vaccine recipients eventually received their second dose just nine days later, so the data does not exist for how long the protection from the single dose would have lasted.

Similarly, for Moderna’s vaccine trial, there appeared to be some protection against COVID-19 following one dose; but the limited data does not provide sufficient information about longer-term protection beyond 28 days after the single dose.

In the absence of supporting evidence, nothing definitive can be concluded about the depth or duration of protection after just a single dose of currently authorized vaccines, or choosing between the studied and longer gaps between doses.

While the efficacy of the mRNA COVID-19 vaccines against symptomatic COVID-19 has exceeded expectations, researchers still do not know how long that protection lasts. In the follow-up of the phase 1 trial of Moderna’s vaccine during the 119 days after the first dose, the antibodies declined in all participants and the neutralizing antibodies – which not only bind the virus but also block infection – fell 50% to 75% in the people older than 56.

What can happen if vaccination is incomplete?

Viruses naturally mutate because of copying errors in their genetic code as they multiply in the host’s body, or due to swapping of genetic codes between different viruses co-infecting the same host.

But they also evolve to evade the immunity of the host, specially if competing against weak but sustained immune response. SARS-CoV-2 can already lie low in infected individuals, and approximately 40% to 45% of those infected display no symptoms at all. In an immunocompromised patient – using therapies to fight autoimmune disease or cancer – the virus has been found to be present for up to 154 days. In such situations there are increased odds that a virus variant can emerge that can escape the immune response and spread fast. Indeed, it is suspected that the new highly infectious U.K. variant, which is also spreading in the U.S., could have originated in a chronically infected individual.

Although evolution of vaccine resistance is considered very rare because of effective and rigorously developed vaccines, mathematical modeling suggests that a resistant virus can readily arise if immune response is too weak to destroy all the viruses in the host.

Rushed and ineffective vaccines can produce antibodies that fail to recognize and bind viruses poorly, which can do more harm than good.

Changing the dosing to overcome supply shortages is a contentious and ongoing debate. However, making wrong decisions without adequate scientific evidence could be counterproductive.

This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Sanjay Mishra, Vanderbilt University.

Read more:

  • Vaccine delays reveal unexpected weak link in supply chains: A shortage of workers

  • How many people need to get a COVID-19 vaccine in order to stop the coronavirus?

Sanjay Mishra receives funding from the National Cancer Institute (NCI) and has previously received support from the National Institutes of Health (NIH).

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