When do you reach group immunity?

That’s a reasonable question. People are asking because they want to know when the blockage will end. When they can reunite with their loved ones abroad. When their business becomes safer. When the heading is no longer dominated by COVID-19.

Currently, expert modelers are plugging in numbers and are examining different scenarios to estimate the extent of protection provided by different levels of vaccination coverage. We look forward to seeing the results of this modeling from the Doherty Institute as early as this week.

However, it is important to recognize that it is difficult to identify a single magic number for herd immunity.

What is herd immunity again?

To understand why experts often avoid identifying the single vaccination count needed to reach COVID-19 herd immunity, let’s first summarize the concept.

Herd immunity is when the immunity of the population is high enough to block the pathway of transmission of an ongoing disease.

Vaccination provides each of us with direct protection against disease and herd immunity, but even unvaccinated people benefit from its blocked transmission pathways.

Different illnesses have different herd immunity thresholds. For measles, for example, the herd immunity threshold is 92% to 94%. Estimates for COVID-19 vary, with some reaching over 85%.

However, many hesitate to give a single number. Here are three reasons.

1. Vaccine variations and the disease itself

If the infectivity of SARS-CoV-2 (the virus that causes COVID-19) continues to fluctuate, it is difficult to estimate a single herd immunity figure.

The infectivity of the disease can be understood by examining R0, the number of reproductions, the average number of people infected in one uncontrolled case. The ancestral strain of SARS-CoV-2 has a R0 of 2-3, but Delta is estimated to be twice as infectious and R0 of about 4-6.

The type of vaccine, the dose given (one or both), and how well the vaccine covers the various variants are all considered.

The UK estimates that a double dose of Pfizer vaccine is 85% to 95% effective against symptomatic disease of alpha variants, and a double dose of AstraZeneca is 70% to 85% effective. Overall vaccine efficacy appears to be reduced by approximately 10 percentage points with the Delta variant.

The less effective the vaccine, the higher the level of coverage required to properly manage COVID.

2. Not yet able to cover the entire population

The Pfizer vaccine is currently tentatively approved in Australia for children aged 12 to 15 years. Vaccinations can take time, even if they are routinely recommended for this age group. Even if that happens, there remains a gap in our population protection among young children.

Children should benefit from adult vaccination to some extent. In the United Kingdom, the overall intake of the vaccine was 48.5% with two doses, but initially the infections in children under the age of 10 were reduced. This is partly due to the indirect protection provided by vaccinated adults.

3. Population protection depends on time and space

There is rarely a decent threshold. Then everything changes forever. Vaccine protection for an individual can decline over time. With the advent of new variants, boosters will almost certainly be needed to maintain population protection against COVID-19.

With influenza vaccination, the duration of prevention is so short that we rarely even talk about herd immunity. By the next flu season, this season’s vaccine immunity will be much less effective against the latest virus strains.

Spatial, protection may vary by region and demographics. Even in countries that have reached the herd immunity threshold for measles vaccination, children have small outbreaks in narrow pockets and a cohort of teens and adults are well vaccinated as children. You may not have received it.

The ability to achieve herd immunity is also affected by population density and the amount of people in the population that mix with a variety of other people, that is, the heterogeneity of the mix.

Life changes gradually as more people are vaccinated

Given these factors, understanding professionals often avoid giving a single number of herd immunity.

The infectivity of Delta requires very high immunization rates. That way, life will look different, especially if this happens globally. Australia can relax border restrictions. People who are fully vaccinated may see modified forms of quarantine, such as home quarantine.

Although COVID outbreaks occur, the risk is low and fewer people are susceptible to serious illness. City- or state-wide outbreaks are replaced by more local ones.

Good public health measures such as rapid contact tracing and quarantine continue to be needed. Rapid testing may be used more often. New treatments may be found.

In the meantime, we need to worry about global vaccine coverage as much as national vaccine coverage. Because everyone, by any means, has the right to freedom and security that comes from COVID-19 protection.

And, as we’ve heard from global leaders, “No one of us will be safe until everyone is safe.”