After registering the best month ever, the energy sector continued its bullish trend as oil price rose on vaccine optimism. The rollout of COVID-19 vaccines soon after the encouraging trial results from Moderna MRNA, Pfizer PFE and AstraZeneca AZN has raised the expectation of a faster-than-expected recovery in energy demand.Britain began Pfizer’s vaccinations this week while Canada plans to roll out vaccination next week after being approved. The Food and Drug Administration also authorized Pfizer's vaccine for emergency use in people aged 16 and older. Additionally, reports showing Chinese demand exceeding pre-COVID levels and an uptick in Chinese refinery runs also led to a spike in oil price. A weak dollar has been supporting oil price as it makes the dollar-denominated commodity less expensive for foreign buyers.The rally came despite the reports showing a sharp rise of 15.2 million barrel in U.S. crude inventories — the largest weekly increase since April — for the last week. In fact, Brent oil surged to above $50 per barrel for the first time since early March (read: Should You Buy Oil & Gas ETFs Now?).Further, the OPEC and its allies reached a fresh deal to curb oil output in January. Though the cartel will curb about 7.2 million barrels per day next month, it is lower than the current 7.7 million barrel per day cuts.Apart from these, the energy market has been benefiting from the positive roll yield in the futures market. This is because the oil market is currently in a state of backwardation, where later-dated contracts are cheaper than near-term contracts, for months. Per CME Group data, Brent futures for March delivery were trading about 13 cents below February contracts. This signals that the oil market is tightening and demand is robust, paving the way for an oil rally. This trend is likely to persist at least in the near term, acting as the biggest catalyst for the commodity.Given this, we have highlighted five energy ETFs that have been at the forefront over the past month.SPDR S&P Oil & Gas Equipment & Services ETF XESThis fund tracks the S&P Oil & Gas Equipment & Services Select Industry Index, which measures the performance of the companies engaged in the oil and gas equipment and services industry.Zacks Rank: #5 (Strong Sell)AUM: $117.3 millionExpense Ratio: 0.35%4-Week Return: 46.9%VanEck Vectors Oil Services ETF OIHThis fund tracks the MVIS U.S. Listed Oil Services 25 Index, which offers exposure to companies involved in oil services to the upstream oil sector, including oil equipment, oil services or oil drilling (read: 5 ETFs at the Forefront of the Latest Market Rally).Zacks Rank: #4 (Sell)AUM: $662.2 millionExpense Ratio: 0.35%4-Week Return: 46.1%iShares U.S. Oil Equipment & Services ETF IEZThis ETF offers exposure to U.S. companies that provide equipment and services for oil exploration and extraction by tracking the Dow Jones U.S. Select Oil Equipment & Services Index.Zacks Rank: #5AUM: $65.8 millionExpense Ratio: 0.42%4-Week Return: 44.3%First Trust ISE-Revere Natural Gas Index Fund FCGThis fund offers exposure to U.S. stocks that derive a substantial portion of their revenues from the exploration and production of natural gas. It follows the ISE-Revere Natural Gas Index.Zacks Rank: #3 (Hold)AUM: $109.5 millionExpense Ratio: 0.60%4-Week Return: 43.6%Invesco Dynamic Oil & Gas Services ETF PXJThis product follows the Dynamic Oil Services Intellidex Index, which offers exposure to companies that are engaged in the drilling of oil and gas wells; manufacturing oil and gas field machinery and equipment; or providing services to the oil and gas industry, such as well analysis, platform and pipeline engineering and construction, logistics and transportation services, oil and gas well emergency management and geophysical data acquisition and processing (read: Energy on Track for Its Best Month Ever: 5 ETF, Stock Winners).Zacks Rank: #5AUM: $8.3 millionExpense Ratio: 0.63%4-Week Return: 41%Want key ETF info delivered straight to your inbox?Zacks’ free Fund Newsletter will brief you on top news and analysis, as well as top-performing ETFs, each week. Get it free >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report AstraZeneca PLC (AZN): Free Stock Analysis Report Pfizer Inc. (PFE): Free Stock Analysis Report Moderna, Inc. (MRNA): Free Stock Analysis Report SPDR S&P Oil & Gas Equipment & Services ETF (XES): ETF Research Reports iShares U.S. Oil Equipment & Services ETF (IEZ): ETF Research Reports First Trust Natural Gas ETF (FCG): ETF Research Reports VanEck Vectors Oil Services ETF (OIH): ETF Research Reports Invesco Dynamic Oil & Gas Services ETF (PXJ): ETF Research Reports To read this article on Zacks.com click here.