Amid efforts to control COVID-19, many people are curious about what proportion of the population should be vaccinated to achieve mass immunity. This is a valid question. People ask because they want to know when they will see the lockdown end, when they can reunite with loved ones overseas, when their businesses will be safer, and when the headlines will no longer be dominated by COVID-19.
Right now, experts are trying to estimate the number and extent of protection that will give us different levels of immunization coverage under different scenarios. We look forward to seeing the results of this Doherty Institute effort earlier this week. But it’s important to recognize that it’s hard to say a single magic number for collective immunity or mass immunity. Let us understand once again what collective immunity is? To understand why experts often avoid stating the number of a single vaccination needed to achieve mass immunity against COVID-19, let’s review the concept first.
92% to 94% resistance required to achieve collective immunity
Herd immunity occurs when the immunity of a population is high enough to block the pathway for continued disease transmission. Vaccination provides each of us with direct protection against disease, with herd immunity preventing transmission of the disease and benefiting those who have not received the vaccine. Different diseases have different thresholds for herd immunity. For example, in the case of measles, resistance of 92% to 94% is required to achieve herd immunity. Estimates for COVID-19 vary, with some putting it at 85% or more.
However, many people are reluctant to give a number. Here are three reasons for this. 1. Variation in vaccines and the disease itself When the infectivity of SARS-CoV-2 (the virus that causes COVID-19) remains so variable, it is difficult to estimate a number for herd immunity. We understand the contagiousness of a disease by looking at the OR, or disease progression, the average number of people infected with a case for which there are no control measures. The parental strains of SARS-CoV-2 have an OR of 2-3, but the delta infection is twice as high, around RO 4-6. The type of vaccine, the dose given (one or both) and how well the vaccines cover the different types are all factors.
British estimates show that two doses of the Pfizer vaccine with the alpha version are between 85% and 95% effective against symptomatic disease, while two doses of AstraZeneca are between 70% and 85% effective. The overall efficacy of the vaccine appears to drop to around ten percent with the delta version. The lower the vaccine’s effectiveness, the higher the level of coverage we need to control COVID. 2. We may not yet cover the entire population. The Pfizer vaccine has now been provisionally approved for 12-15 year olds in Australia. Although it is systematically recommended for this age group, it will still take time for them to be vaccinated.
Booster needed to maintain immunity against covid
Even once that happens, the gap in the safety of our population for young children will persist. Children should benefit to some extent from adult vaccinations. In England, where a total of 48.5% received the full vaccine with two doses, there was initially a decline in infections in children under ten. This is in part due to the indirect protection offered by adults. 3. The immunity of the population will vary over time and space. Such an exact limit will rarely be found, after which everything changes for good. Vaccine protection in individuals is expected to decline over time. With the arrival of new variants, we will certainly need boosters to maintain the immunity of the population against COVID-19.
With influenza vaccination, we rarely talk about mass immunity, because the period of protection is so short. Until the next flu season, the immunity of the current season’s vaccine will be much less effective against the last strain of the virus. Generally speaking, security may vary by location and demographics. Even in a country where measles vaccine coverage has reached the required collective immunity threshold, you may see small outbreaks of the disease in children in areas with low coverage, or when a group of adolescents and children adults are treated like children. ended.
The ability to acquire herd immunity is also affected by population density and the number of people in a population mixing with different types of people – this is called mixing heterogeneity. Life will gradually change as more and more people get vaccinated. Considering these factors, it’s understandable that experts often avoid giving a single number for mass immunity. With Delta’s infectivity, we will need much higher vaccination rates. Then again, life will be different, especially when it takes place on a global scale. Australia will be able to relax its border restrictions. We will likely see modified forms of isolation, such as home quarantine, for those who have been fully vaccinated.
Everyone has the right to be safe from Kovid-19
There will be outbreaks of COVID, but they will be less risky, fewer people will be susceptible to serious illnesses. City or statewide outbreaks will be followed by local outbreaks. We will always need good public health measures like early contact tracing and isolation. Rapid tests can be used more frequently. New treatments can be found. At all times, we must be as concerned about global immunization coverage as we are about national coverage. Because everyone has the right to be free from COVID-19. And as we have heard from world leaders, “none of us will be safe until all are safe”.
Julie Lisk, professor at the University of Sydney, and James Wood, academic in public health, UNSW