New Delhi, Nov 24 : The positive readouts from the vaccine trials mean that the United States will most likely reach an epidemiological end to the Covid-19 pandemic (herd immunity) in Q3 or Q4 2021, according to a research by McKinsey & Co. The report said an earlier timeline to reach herd immunity - for example, Q1/Q2 of 2021 - is now less likely, as is a later timeline (2022). "If we are able to pair these vaccines with more effective implementation of public-health measures and effective scale-up of new treatments and diagnostics, alongside the benefits of seasonality, we may also be able to reduce mortality enough in Q2 to enable the United States to transition toward normalcy," it added. There can be early herd immunity if vaccine rollout and adoption is faster than expected and natural immunity is significantly higher than realized. The peak probability of her immunity by Q3/Q4 of 2021 is driven by emergency use authorization (EUA) of one plus candidates in December 2020/January 2021. Biologic license application (BLA) (with full approval by March/April 2021) and approximately six months for manufacturing, distribution and sufficient adoption to reach herd immunity. An epidemiological end point will be reached when herd immunity is achieved. One end point will occur when the proportion of society immune to Covid-19 is sufficient to prevent widespread, ongoing transmission. The report said many countries are hoping that a vaccine will do the bulk of the work needed to achieve herd immunity. When this end point is reached, the public-health-emergency interventions deployed in 2020 will no longer be needed. While regular revaccinations may be needed, perhaps similar to annual flu shots, the threat of widespread transmission will be gone. A second and likely, earlier end point, a transition to normalcy, will occur when almost all aspects of social and economic life can resume without fear of ongoing mortality (when a mortality rate is no longer higher than a country's historical average) or long-term health consequences related to Covid-19. The process will be enabled by tools such as vaccination of the highest-risk populations; rapid, accurate testing; improved therapeutics; and continued strengthening of public-health responses. The next normal won't look exactly like the old-it might be different in surprising ways, with unexpected contours, and getting there will be gradual-but the transition will enable many familiar scenes, such as air travel, bustling shops, humming factories, full restaurants, and gyms operating at capacity, to resume. A secondary effect of the recent vaccine trials is to make Q3 2021 more likely for herd immunity than Q4. That said, major questions are still outstanding, even about vaccines, such as long-term safety, timely and effective distribution, and vaccine acceptance by the population, to say nothing of lingering epidemiological questions such as the duration of immunity, the report added. The world has cheered announcements over the past two weeks by Pfizer and its partner BioNTech, and from Moderna. Their Covid-19 vaccine candidates are showing efficacy rates that are higher than many dared hope for. At about 95 per cent, efficacy is higher than expected by most experts. "Caution is still warranted. The safety records of the Pfizer and Moderna vaccines appear promising so far (no serious side effects reported), but the coming months will provide a fuller picture as the sample size grows. We don't yet know how long the protection the vaccines offer will last. The Pfizer trial has enrolled some children (ages 12 and older), but efficacy in those under 18 remains unclear," the report said. /IANS