As questions swirl about what those mutations mean for the variant’s transmissibility and possible resistance to vaccines, Fortune asked three leading COVID-19 experts—Ben Cowling, chair of epidemiology at the University of Hong Kong; Catherine Bennett, chair of epidemiology at Deakin University in Australia; and Nicholas Thomas, a professor of global health governance at the City University of Hong Kong—how concerned they are about Omicron and what they have learned about the new COVID-19 strain so far.

The three experts say the variant’s mutations suggest it may be able to fend off some of the protections offered by vaccines, but they caution that it’s too early to fully grasp the risks posed by Omicron. They say that for now the best defense against this variant and potential future strains are the COVID-19 jabs currently on the market.

Fortune: What do we know so far about Omicron?

Cowling: This variant is increasing in incidence in the southern part of Africa, particularly in South Africa. And it’s got a lot of mutations compared to, say, the Delta variant or other variants that we’ve seen before. But to what extent is this virus more transmissible than Delta or other variants? It can certainly spread, but to what degree? I also don’t think we have a good handle on its severity.

Bennett: The mutations have raised levels of concern about how they might impact how infectious [Omicron] is, and how effective our current vaccines are against it. [Health officials have] reported quite a rapid escalation of cases, particularly in South Africa. We’ve also seen it spread pretty quickly with travelers around the world, and some of these people are actually fully vaccinated. So that’s another suggestion that maybe it’s a bit more infectious.

Does the Omicron variant lead to milder COVID-19 infections?

Cowling: [A doctor in South Africa] is saying that it is mild. But that’s not data. That’s just a comment from someone. I don’t know what to make of that. I think we’re all waiting to have more clear data on severity.

In South Africa, my understanding is that the majority of the population has been infected in previous [outbreaks]. And there could be people who’ve been infected more than once. And so the severity of Omicron in South Africa may not be the same as the picture of severity in other places where there’s maybe a lower chance that people have been infected but a higher chance of being vaccinated. (South Africa has fully vaccinated 24.3% of its population, according to Bloomberg.)

Bennett: The early information about severity of illness is reassuring; we’re not hearing anything that really alarms us. If [Omicron] causes less severe illness, then that’s a big relief, and it means that we’re probably less concerned about this variant.

Thomas: If we assume that this is a more transmissible variant, but it doesn’t produce as dangerous or lethal responses, then you’re looking at people getting sick, but not necessarily requiring hospitalization. The problem is we still have large segments of the population that are not only unvaccinated, but also have a range of comorbidities. They are already reeling from Delta. Those more vulnerable populations are the ones we need to be concerned about.

Dr. Anthony Fauci of the U.S. National Institutes of Health said booster shots are the most effective defense against Omicron. Do vaccines still work against Omicron?

Cowling: Our understanding of immunity is that even if there’s a mismatch in the strains, vaccines will provide some defense. Maybe in early 2022, we could hope that there might be updated Omicron vaccines, but those take time to develop and test and then produce.

Bennett: We should stay with the vaccine schedule that we had planned with Delta until we learn more about this new variant. We don’t want to take our sights off Delta, or focus only on Omicron, and then find out that [Omicron] is quite benign. We just keep doing what we’re doing [on vaccine campaigns] until we learn anything different about how we might use boosters to give us better protection against this new variant.

Thomas: [The Omicron variant] reinforces the case that we need to have booster shots. In being well vaccinated, we have a chance to disrupt disease transmission chains. The longer we allow those transmission chains to go on, the greater the risk that mutations arise that are resistant to existing vaccine technologies.

Can a new variant like Omicron change the efficacy of forthcoming COVID antiviral pills?

Cowling: [For vaccines], it’s quite straightforward to have an approximation of how well they’ll protect against different variants because we have the blood samples. We can go back and test [blood samples] for antibodies to the Omicron variant and see to what extent there are cross-protective antibodies. For antiviral drugs, there’s no compatible test. We really have to test it in the field [to see if the antiviral pills work against Omicron].

Bennett: It’s too early to tell. But if [Omicron] is a mild illness, particularly one that isn’t, at least from what we’re hearing so far, presenting as a respiratory illness, then that actually takes away the concern that an infection will escalate to serious disease [that an antiviral pill could help treat].

Should countries be closing their borders to guard against Omicron’s spread?

Cowling: I can understand [border closures] if they are being put in place right now to buy time for decisions. But I doubt that those those kinds of measures would be very sustainable. We certainly know that travel restrictions cannot provide watertight protection against a new virus or a new strain of a virus. At best they can delay things for maybe a week or two.

Bennett: I do think [border closures] are sensible until we understand more about this particular variant. Countries will want to keep [Omicron] at the border as long as they can, so that they can then decide what is the appropriate way to manage it, once we understand it more. Controlling direct flights into a country [buys the country time to impose] really good testing protocols and isolation periods for positive [passengers] if they don’t already have them in place.

Thomas: If everyone closed their borders, this wouldn’t get out. But the trouble is, not all countries are closing their borders. Most countries, with the exception of places like China, are trying to keep their borders open but just not to those countries that are most directly affected by this variant. People are going from those affected countries to other countries, and then can come into the main country. It’s that sort of fragmented secondary transmission route where a lot of these border closures just don’t make sense. A real danger here is that people will overreact to this, and then not react so strongly when a [potentially more deadly] variant comes along.

Is Omicron worrisome enough that people should cancel their Christmas plans?

Cowling: I think people are going to be thinking very carefully about their travel plans because of the potential for disruption and not wanting to be stuck somewhere [if borders close]. But it’s still early days, and we are still waiting for more information on the variant.

Bennett: Not at this stage. So far, the signal is that [Omicron] is not likely to be one [variant] that we have to be overly anxious about. But we need to be sure of that before we start to step back and open up international borders more freely. At the end of the day, we have to get used to this, this is going to be what living with the virus looks like.