A more powerful driver of Covid-19 outbreaks than any strain of the coronavirus encountered so far, the delta variant is testing the limits of public health defenses globally. Delta is both more infectious and more capable of evading immunity generated by either vaccination or a previous natural infection. It’s not known exactly why, but scientists believe it’s probably a combination of factors, including that:
- It replicates faster and reaches significantly higher concentrations in the upper airways.
- It’s expelled in greater quantities by infected individuals.
- Its viral particles are better at latching on to the ACE2 receptor, an enzyme found on the surface of many cells that the virus uses to infect them.
- It may be more efficient at causing infection, by requiring fewer viral particles or a lower infectious dose.
Are vaccines working?
Yes. The benefit of immunization is that it trains the immune system to recognize and fight the virus faster, which helps prevent an infection from progressing. So an infection that might otherwise have caused severe illness is more likely to result in a mild case, and what could have been a mild case may instead be an asymptomatic infection. Countries with high rates of immunization have a lower proportion of Covid cases requiring hospitalization, providing proof that vaccines are working.
Then why are infections occurring among the vaccinated?
No Covid vaccine provides complete protection against infection. Antibodies trained to block or neutralize the coronavirus are the key defense against stopping infection and preventing onward transmission. But people react differently to vaccination, resulting in a wide variation in the amount and quality of the antibodies they generate after an inoculation. Research suggests that some vaccinated individuals may not produce sufficient antibody levels in their upper airways to counter a delta infection early enough to stop it from replicating in the nose and throat, leading to so-called breakthrough infections. Still, a fully vaccinated person typically clears the infection faster than someone who’s unvaccinated or partially vaccinated. That not only prevents the infection from causing worse symptoms, it also shortens the period a vaccinated person is likely to be infectious and thus reduces their likelihood of passing the virus on.
Will vaccines ever be able to prevent coronavirus infection?
Potentially. The shots we’re using now may not be the ones we rely on in the future. Among the Covid vaccines under development are some that aim to stimulate a more robust immune response in the nasal tract through which the virus is likely to enter the body. The hope is that by improving immune defenses there, the virus could be snuffed out before it has a chance to cause an infection, much less penetrate the lungs and cause a more severe illness.
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