Projecting timetables for widespread distribution of Covid-19 vaccines in the U.S. and around the world is rife with uncertainty, even if the products presented prove reliably safe and effective. Given the many challenges, the process is unlikely to be as quick as many want to believe.

In light of the positive trial results announced this week by Pfizer Inc. [NYSE: PFE] and German partner BioNTech SE [NASDAQ: BNTX], it seems the U.S. and Western Europe will be out with approved vaccines possibly in mid-2021. Emerging markets are likely to follow in 2022 or thereafter.

Pfizer reported that its COVID vaccine achieved 90% effectiveness in preventing symptomatic disease. The rate of effectiveness is clearly above expectations, and it should help to drive a higher rate of vaccine acceptance. Pfizer, however, needs to continue the trial to accrue more safety data. This data validates the spike protein as a vaccine target, which suggests that other programs, particularly that of Moderna [NASDAQ: MRNA], are likely to work, as well. Each has a promising two-part vaccine in trials of 30,000-50,000 people.

The U.S. Food & Drug Administration (FDA) is spending tremendous amounts of time working on treatments and vaccines for Covid-19. Communications during the pandemic have not been uniform across the agency, but we have gleaned hints as to how approvals could progress.

Many other highly capable companies are also working on new vaccines, including Johnson & Johnson [NYSE: JNJ] and AstraZeneca (NYSE: AZN]. With such ample resources devoted to solving the problem, equally good—if not ultimately superior—alternatives could still emerge.

The key factors for regulatory approval are the vaccine’s effectiveness along with ease of manufacturing. The logistical challenges will be substantial as well, given the potential need for two doses for every recipient. Nevertheless, if these elements fall in line, emergency use authorizations could be issued in the U.S. as early as December.

Here’s the most important thing: Based on the initial results of Pfizer’s trial, it appears their vaccine is effective in preventing symptomatic disease. This could allow vaccinations of those at highest risk, including frontline medical personnel as well as nursing home workers and residents. After that, inoculations will go successively down through the age ranges, with the elderly and highest risk first and the youngest last.

Broad inoculation could begin in the first quarter of 2021 but the second quarter is more realistic. For full inoculation and the return of an unfettered U.S. economy, investors should look out to the third or even fourth quarters of 2021 at the earliest.

Vaccines and treatments will take longer to reach emerging markets. It could be 2022 or even 2023 before we see substantial vaccination rates worldwide.

But developing the vaccine and manufacturing it are only two of the elements necessary to stop the spread of the virus. This element is equally important: Governments and manufacturers must emphasize that only vaccines that have been deemed safe and effective will be released for broad use.

There is significant question over whether governments—particularly in the U.S., where the issue is often raised—can mandate that citizens receive the vaccine. Typically, mandates come at the local level—think elementary schools that require children to receive the measles, mumps and rubella vaccine, for example.

We know that vaccines work most effectively when high numbers of people are inoculated, leaving few hosts who can transmit the disease. This creates herd immunity. If too many refuse inoculation, concerted efforts may be necessary to convince people to line up for them.

Medical facilities and other workplaces with large concentrations of older or at-risk people may require that employees get vaccinated. Large U.S. employers may not make it a requirement but they could strongly encourage employees to do so, or they could prevent the unvaccinated from coming into their workplaces.

Will this lead to a high percentage of children in the developed world being inoculated by the end of 2021? Will schools be able to fully open?

If most U.S. residents are vaccinated by then, the end of uncertainty surrounding the spread of the virus will, I hope, lead to a full economic recovery in the developed world.

Any information, statement or opinion set forth herein is general in nature, is not directed to or based on the financial situation or needs of any particular investor, and does not constitute, and should not be construed as, investment advice, forecast of future events, a guarantee of future results, or a recommendation with respect to any particular security or investment strategy or type of retirement account. Investors seeking financial advice regarding the appropriateness of investing in any securities or investment strategies should consult their financial professional.

Marshall Gordon is director and senior research analyst for health care at ClearBridge Investments.