What Does Herd Immunity Even Mean?
(Bloomberg Opinion) -- The emergence of the fast-spreading delta variant has pushed the threshold for attaining “herd immunity” from the virus to more than 80% from previous estimates of as low as 60%, officials at the Infectious Diseases Society of America said Tuesday. Sam Fazeli, a Bloomberg Opinion contributor who covers the pharmaceutical industry for Bloomberg Intelligence, answers questions about this development and the concept of herd immunity. The conversation has been edited and condensed.
What is herd immunity and how is it achieved?
The simplest explanation is this: enough people have an immunity to the virus, either through vaccination or prior infection, that the chain of transmission breaks. In this way, the whole community is protected. And what does “enough people having immunity” mean? That's the $64 million question, with a complex answer that varies from virus to virus, or better put, from variant to variant when it comes to Covid-19. Going deeper, the level of immunity required to hit the magic “herd immunity” level, in its simplest form, is determined by the following, often interrelated, factors: how easily a virus is transmitted from person to person in the absence of immunity; whether the immunity, be it from a vaccine or prior infection, in an infected individual is good enough to lower viral load so much that they don’t transmit the virus; whether the virus changes in response to our immunity; the degree to which immunity may wane after an infection or vaccination; and the smoothness of vaccine rollout – i.e., if vaccinations are low in one area, there’s a risk that the virus could be brought back into a community that had achieved herd immunity.
At one point, it was thought about 70% immunity could get us out of Covid. What is it about delta that made those estimates go up so much?
Actually, Israel looked as if it had nearly eliminated the virus in June, when its vaccination rate was just about 60%. But then the delta variant came along and things changed. Why? Because the delta variant changes a few of the elements that go into the formula. First, it infects at a much higher initial viral load, meaning it is much easier to transmit from person to person. Next, it is slightly harder for antibodies generated by the vaccines to neutralize it. Third, the initially high amount of virus that an infected person is confronted with (the viral dose) may be so high that there isn't enough immunity in the respiratory system of an immune person to block it (note, though, that this still needs to be proved). Finally, the amount of antibodies in some people who were vaccinated six months ago may have diminished. All of this feeds into one outcome: It is easier to be infected with the delta variant. This is why you now hear about the need for higher immunity thresholds to achieve herd immunity.
Can herd immunity end the threat of Covid-19?
I am going to say yes — but it all depends on what you define as the “threat of Covid-19.” I am not sure I have ever believed that we can actually get to herd immunity against infection, but we can get much closer to the point against moderate to severe disease. The point is, as we develop immunity to the virus — and the best and safest way of getting it, by the way, is through vaccination — the virus can change to overcome the immunity enough to allow it to infect. So we will likely have a level of infection going on for a long time, with the virus becoming “endemic” - the level of which depends on the variables I mentioned above. Also, not everyone responds to vaccination in the same way; some achieve much better immunity than others. But protection against disease is likely to be much higher than protection against infection, which is exactly what we are seeing in the emerging data. That is still a good result.
If herd immunity can't end the pandemic, can it help lead to a widespread ability to live with Covid-19?
Yes — the same way we “live” with influenza or respiratory syncytial virus (RSV), both of which cause a lot of infections but are lethal in some people. Living with it may also require some people to change their behavior, especially if the virus evolves into something even more easily transmissible or lethal.
Can prior infection alone do the job, or are high rates of vaccination needed?
Both. The virus has the ability to tune down our immune response such that it may not be protective for long. So regardless of past infections, vaccination - which carries significantly lower risks than catching the virus - is the best way to equip the body against the virus's more serious effects.
Does the type of vaccine matter?
It seems so, but it’s still early. Current data suggest the mRNA vaccines from Pfizer Inc.-BioNTech SE and Moderna Inc. are the best, both from a safety and efficacy perspective, followed by the adenoviral vaccines from Johnson & Johnson and AstraZeneca Plc. I haven't seen enough published data to judge Russia’s Sputnik shot or the Cansino, Sinopharm or Sinovac vaccines from China. There is also emerging data that shows mixing vaccines may provide better immunity in the respiratory tract, but we need to see much more research on that. I am sure we will hear a lot more about this.
Could boosters help get the job done?
Sure. If you keep topping up people’s antibody levels, they will have a stronger shield against the virus and have a higher chance of stopping it from establishing a foothold - i.e., infecting them. But a serious debate is needed about giving boosters within highly vaccinated richer countries when many lower-income countries haven't even hit initial vaccination rates of 10%.
This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.
Sam Fazeli is senior pharmaceuticals analyst for Bloomberg Intelligence and director of research for EMEA.
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