Since early 2020, governments around the world have adopted numerous public health and social measures to contain and control the coronavirus pandemic. Among the most drastic policies have been border controls, which have had significant economic and social effects and brought international travel to a near halt in 2020. In recent months, most countries have largely reopened borders, especially to those who are vaccinated, leading to significant increases in international travel.

Now, reports of a new coronavirus variant called omicron have pushed many countries to reimpose border measures in hopes of keeping it out. The United States, Britain, the European Union and other nations announced in the past few days that they would ban or require quarantines for flights or travelers coming from countries in southern Africa, including South Africa, where scientists first sequenced the new variant. Others, such as Switzerland, have imposed measures on all locations that have thus far detected the variant.

But we know that border measures aren’t always useful. And it’s highly unlikely that this recent round of targeted restrictions will contain the omicron variant.

As part of the Pandemics and Borders research project, my team and I have conducted research to understand what types of border measures have been effective and in what contexts. This includes a systematic review of the effectiveness of travel measures and ongoing studies that are looking at why countries have adopted these measures. While we have found that travel measures have been extremely effective in some international contexts, most only reduce, but do not eliminate, imported coronavirus cases.

When people discuss border control measures, there is a tendency to refer to such actions as “closing the border.” While a small number of countries — usually very small, isolated ones such as Kiribati — have effectively closed their borders to all incoming travelers during the pandemic, few countries have completely sealed themselves off. More commonly, countries have adopted a variety of measures: issuing travel advice, requiring proof of vaccination or diagnostic testing, even requiring travelers to spend lengthy periods in quarantine. Countries also vary in terms of who they target with these measures: all travelers, those from specific places or just foreign travelers. Effectiveness of the types of measures used during the pandemic therefore varies by international context.

Looking at the effectiveness of travel measures imposed during the first few months of the pandemic has shown that travel measures imposed early were able to slow the initial export of cases outside of China. While it is difficult to fully understand what happened early in 2020, since our ability to test for the virus was imperfect at the time, we know that the widespread lockdown of citizens in Wuhan and the additional flight bans and restrictions imposed by some countries against travelers from there or other places in China led to measurable reductions in the number of cases exported and the time it took for the virus to spread internationally. However, the virus spread faster than the targeted restrictions countries had imposed, and soon imported cases were coming from places outside of China, such as Iran and Italy, which were not subject to travel restrictions.

Targeted travel restrictions, such as those currently imposed on southern African countries, are effective only at preventing cases from places where the virus has been detected — not necessarily where it is now, and certainly not where it will be in the future. Genetic sequences of the omicron virus had been shared as early as Nov. 11, the same day a traveler from South Africa carrying the variant strain arrived in Hong Kong. That is strong evidence that omicron had already been in circulation at least two weeks before the latest travel measures were imposed. In the past few days, cases have also been detected in Belgium, Britain, Israel and halfway around the world in Australia. Omicron has achieved frequent flier status, probably, with numerous airlines.

A second set of lessons comes from looking at previously successful “Zero Covid” places, including Australia, Singapore, China, Hong Kong, New Zealand and Vietnam. All these places had adopted a common set of measures: lengthy quarantines for all or most incoming travelers in a dedicated facility, pre-departure and/or upon-arrival PCR testing, and some form of restriction on who can or cannot enter. Most of these locations also put the core elements of their border control measures into place in March or April 2020, although fine-tuning of these measures continued over many months. Used in combination, these measures were highly effective at keeping the coronavirus out of these locations for most of the pandemic. Strict border control measures have already detected and isolated the first cases of omicron in Hong Kong, preventing it from entering the community.

Although methodologically it is difficult to pinpoint the independent effect of each measure, the long incubation period of the coronavirus means that quarantines of about 14 days are probably the most important component to keep the virus out — and the most successful places have applied these measures to all inbound travelers, with few or no exceptions. Travel measures that apply only to some people and not others make border control measures less effective. The United States has restricted travel for noncitizens from eight southern African countries — but U.S. citizens are still allowed to return from these places. Britain is requiring people to take a PCR test within two days of their arrival, and travelers are only required to self-isolate until the test comes back negative. While these measures will certainly be disruptive to international travel, they will do little to prevent the importation of the virus.

Not only did the former Zero Covid places put in place strong border measures, but they also imposed strong domestic public health measures. Hong Kong tests hundreds of thousands of people every month. Australia imposed some of the longest and strictest lockdown measures in the world. Border control measures should not be seen as a separate strategy — they’re an integral component of the overall national response. They reinforce the effectiveness of domestic public health measures, but they are insufficient to control localized epidemics on their own. Paradoxically, the value of border control measures increases when there are fewer local cases. Unless countries that are reimposing border control measures also reimpose strong domestic measures, such as mask mandates and social distancing, even if a single case is imported, then border control measures will do little to stop the spread of the virus in the community.

The pandemic has challenged much of what we believed to be true about the effectiveness of public health measures, as well as our ability to comply with such measures. And as tired as we are of battling against this virus, it is not done with us yet. New variants will pose an ongoing challenge. As effective as some travel restrictions put in place by some countries have been, it is unlikely that the current round of travel measures being reimposed will do much to prevent the spread of this variant of the virus. Instead, we need to work harder to ensure high rates vaccination everywhere — but especially in low-income countries where vaccines are still in short supply. And we also must continue to maintain domestic public health measures to help control community transmission. It is not until then that we can begin to see the end of this pandemic.