Why Delta is making herd immunity harder to reach

Many countries are counting on vaccines to build sufficient immunity in their populations so that SARS-CoV-2, the virus that causes Covid-19, isn't able to find enough people to infect, causing transmission to eventually stop. But even in countries with a high proportion of people inoculated with highly effective vaccines, it's unclear whether it's possible to reach the so-called herd immunity threshold anytime soon. Researchers warn that the virus is apt to be circulating among us for a long time, although it's likely to become a less potent foe.

1. Can Covid be eradicated?

No. So far, only one human disease -- smallpox -- has been officially eradicated; that is, reduced to zero cases and kept there long-term without continual countermeasures. That was thanks to a good vaccine plus the fact that humans are the only mammals naturally susceptible to infection with the smallpox virus. By contrast, many species are susceptible to SARS-CoV-2, including bats, minks, cats and gorillas. The next best thing would be what's known as disease elimination. That's when there are zero new cases in a defined area over a sustained period, such as 28 days. Some countries, such as New Zealand, have achieved zero new cases for lengthy periods using lockdowns, diligent case detection and isolation, and border closures. But keeping this up over the long run is challenging as the emergence of more-infectious variants leads to even stricter public health and social measures, while people hunger for a return to normal life.

2. Will vaccines eliminate Covid-19?

There's considerable uncertainty about that. One scientific paper calculated that if a vaccine could provide a lifelong, fail-safe shield against infection with SARS-CoV-2, it would need to reach 60% to 72% of people to establish herd immunity. But if a vaccine is only 80% effective at preventing any infection, 75% to 90% of people would need to be immunized -- a high bar. The Covid vaccines in use today have been shown to offer 50% to 97% protection against becoming sick, but it's mostly unknown how well they prevent people from getting an infection without symptoms that could still be passed on. The rapidly spreading delta variant, which is about twice as infectious as the original strain, is weakening vaccine effectiveness too. Another question is the duration of protection. The shorter it turns out to be, the higher the rates of immunization required to establish herd immunity. An unpublished study by Pfizer Inc. researchers that followed vaccinated individuals for as many as six months reported a gradually declining trend in vaccine efficacy against a symptomatic infection, but found their shot remained "highly efficacious" overall. Vaccines might not have to do all the work to get there: Some people who've already had the virus will be immunized against it, although not as well as those who've been inoculated.

3. How do variants of the virus factor in?

The more the coronavirus circulates, the more opportunity it has to mutate in ways that enhance its ability to evade the immunity people have gained. Over the past year, such variants have spread internationally, leading to new surges in cases and hospitalizations. Studies from the UK have shown delta, discovered in India in October, is more resistant to vaccines than the alpha variant that emerged in England in late 2020, particularly in people who have received just the first of two doses. Again, that means a higher proportion of people in the community will have to be immunized to reach herd immunity. Scientists say the vaccines should still work at stopping severe disease in the vast majority of cases, but some shots may be less effective at protecting against mild infections. Inoculations may need to be updated periodically to maintain their efficacy, and several countries plan to roll out additional or "booster" shots, though the World Health Organization called for a moratorium on them until at least the end of September to enable poorer countries to catch up in vaccination rates.

4. Why is delta especially problematic?

Vaccines have been shown to reduce the concentration of virus particles, or viral loads, in the airways of individuals who become infected, lowering the probability of them transmitting their infection to others. Delta, though, is associated with viral loads that are more than 1,200 times higher in newly infected individuals compared with the original coronavirus strain -- a feature that helps delta spread as easily as chickenpox. Worryingly, health officials in a county in Wisconsin in which more than two-thirds of the population is fully vaccinated and delta is the predominant strain, found no difference in viral load regardless of whether the infected person was inoculated. That may help explain a dramatic outbreak in Provincetown, Massachusetts, in mid-to-late July, in which about three-quarters of the 469 Covid cases detected occurred in fully vaccinated people. The U.S. Centers for Disease Control and Prevention recently recommended people return to wearing masks indoors in some situations. It considers delta more prone to breaking through vaccine-induced immunity and says it may cause more severe disease than all other forms of the virus, the New York Times reported in late July.

5. Do Covid vaccines have to prevent infection to curb cases?

No. Vaccines don't have to be perfect to have a public health benefit. New Zealand vaccinologist Helen Petousis-Harris points to rotavirus and chickenpox as examples of diseases that have been "virtually eliminated using vaccines that are very good at preventing severe disease, quite good at preventing any disease, but that do not completely prevent infection in everyone." It's anticipated that newer-generation vaccines will be even better at preventing onward transmission. In the meantime, Mike Ryan, head of the WHO's emergencies program, suggests that rather than focusing on elimination, success should be seen as "reducing the capacity of this virus to kill, to put people in hospital, to destroy our economic and social lives."

6. What if Covid isn't eliminated?

In a poll of more than 100 scientists conducted by the journal Nature in January, nearly 90% said they expected SARS-CoV-2 to become endemic. Viruses that are endemic continuously circulate in the community, often causing periodic spikes when conditions favor transmission. Examples include norovirus, the notorious cause of gastroenteritis on cruise ships, and the myriad of viruses, including four coronaviruses, that cause the common cold.

7. What might the implications be?

People who have survived Covid and those vaccinated against it will probably have some degree of immunity for some time. UK researchers say it's possible that, as immunity gradually wanes, more vaccinated individuals will get infected, but their immune system will rapidly control the infection, resulting in no or mild symptoms. However, exposure to the virus will further boost and broaden their immune response to it. In the absence of herd immunity, the virus will continue to find those who don't have protection. That will mean people who don't get vaccinated -- or can't because their immune systems are compromised, or they have allergies to vaccine ingredients, or are too young -- will remain vulnerable. Some scientists have predicted that, once the endemic phase is reached, then primary exposure to the virus will occur in childhood, when the infection mostly causes mild or no disease. At that point, Covid may be regarded much like the common cold.

Disclaimer: This article first appeared on Bloomberg, and is published by special syndication arrangement