What we know about Covid-19 seems to change by the minute.

It’s understandable, given the massive scale of the global pandemic. Right now, there are more than 85 million known cases around the world and counting. The virus is also relatively new to the medical world, so researchers are learning as they go.

The amount of information out there is dizzying. It’s hard to keep track of what’s known, what’s a myth and what guidance we should follow. That’s why we’ve rounded up five of the most important new things we learned about Covid-19 in December:

1. New, more contagious strains are circulating.

It was big news last month when health officials identified a new and apparently more contagious strain of the SARS-CoV-2 virus here in the UK. There’s evidence that this strain (official name is B.1.1.7) has spread widely. By early 2021, it had been identified in California, Colorado, New York and Florida.

There are many unknowns at this point, but it does seem clear that the UK strain is more contagious than the standard form of the virus (56% more transmissible, according to preliminary study).

However, B.1.1.7 does not appear to make people sicker than the predominant strain, or increase individuals’ risk of death. 

It’s not the only strain being closely watched. In December, health officials identified another variant of SARS-CoV-2 circulating in South Africa (official name 501Y.V2). Like the UK strain, it appears to be more contagious but does not necessarily appear to make people sicker. 

These new strains aren’t unexpected. “RNA viruses, they make a living out of mutating,” Dr. Anthony Fauci, the nation’s top infectious disease expert, has said. The Centers for Disease Control and Prevention states that SARS-CoV-2 acquires one new mutation in its genome roughly every two weeks, although those mutations do not necessarily change how the virus functions.

And at this point, it looks like existing Covid-19 vaccines will still protect against the UK strain — although health officials have sounded the alarm over whether vaccines will stand up to the South Africa variant.

Which is why health officials like Fauci are doubling down on the importance of proven public health measures. “The wearing of masks, the physical distancing, the avoiding crowds and congregate settings — those are the kind of things that will prevent this new strain from spreading even further,” he has said. 

2. Allergic reactions to the vaccine are possible — but the risk is low. 

As the Covid-19 vaccines began to slowly roll out last month around the world, reports surfaced of people who experienced allergic reactions to both the Pfizer/BioNTech and Moderna options.

Some reactions have been severe, with a handful of individuals experiencing anaphylaxis. But the CDC says it has also been notified of reports of individuals experiencing milder reactions, including hives and wheezing.

Experts emphasise that such reactions are rare. “With large numbers of people being vaccinated, reports of allergic reactions are very rare and have been easily treated with resolution,” Daniel Griffin, chief of infectious disease at ProHEALTH Care and national process expert for Optum Vaccine Administration, told HuffPost in December. 

The CDC now urges anyone who has had an allergic reaction to other types of vaccines in the past to consult with their doctor before they consider the Covid-19 vaccine. The group also states that anyone with a known allergy to polyethylene glycol (PEG) or polysorbate should not get a Covid-19 vaccine. 

3. People who get vaccinated might still be infectious.

Research trials typically show whether vaccination prevents individuals from spreading the virus to others, but given the extremely fast pace of vaccine development last year, that remains unclear.

The question has pretty significant implications for our collective ability to curb transmission, because if the vaccine stops people from spreading the virus to others we’re likely to get herd protection much sooner than if it only prevents the vaccinated individual from developing symptoms themselves.

For example, researchers with the Fred Hutchinson Cancer Research Center worked out a model of the Puget Sound region (around Seattle) that showed if the two existing Covid-19 vaccines do make individuals non-infectious to others, a fourth wave of the virus in that area could be avoided altogether. If the vaccine simply prevents symptoms in the individual who received it, there’s likely to be a fourth wave of about 200,000 new infections in 2021.

Again, that’s just one preliminary model. But it shows that the stakes to the question of whether Covid-19 vaccine recipients are still infectious are pretty darn high. 

4. Pregnant and breastfeeding women should get vaccinated. 

In December, the American College of Obstetricians and Gynecologists provided the first official guidance about Covid-19 vaccination for women who are pregnant or breastfeeding, concluding that they should be vaccinated (if they otherwise qualify) despite the fact that no research has been done to date on that particular population. 

In the UK, pregnant women are able to have it if they have an underlying health conditions – read more in our guide below. 

ACOG says that women should absolutely have access to all of the information that’s available to them on vaccine safety in pregnancy (again, at this point, not much) and that they should be encouraged to talk to their health care providers about their options — but that having a conversation with a doctor or midwife beforehand is not a must.

In other words, there should not be additional roadblocks for pregnant women who want to get vaccinated. 

5. The virus might be able to directly enter the brain.

While much of what we learned about Covid-19 in December centered around vaccination, there’s still plenty of research happening about how exactly the virus functions. A highly preliminary animal study published in the journal Nature Neuroscience found that the virus’s spike protein is able to cross the brain-blood barrier — suggesting that the virus as a whole may be able to do so as well.

But even if it cannot, the study may offer some explanation for why people infected with Covid-19 experience a range of neurological symptoms, including brain fog, or “Covid dementia.” When freed spike proteins cross the semi-permeable barrier that protects the brain from outside invaders, they may then circulate freely within the brain, throwing things out of balance. 

And overall, it reflects an understanding of just how far-reaching the impact of Covid-19 can be. It does not simply target the lungs; it can impact virtually all organs in the body. 

“The central nervous system is likely to be a very significant player in how we treat our patients,” said William Banks, a professor of medicine at the University of Washington School of Medicine and researcher behind the December animal study said in a short YouTube video describing his team’s findings. 

Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus.